Fundamentals of Inorganic Chemistry [7] 9789389017564, 9788123923543, 9885175004, 9623451994, 9021734563, 9334159340, 9000660880

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Volume 1 Atomic Structure; Wave Mechanics and Quantum Chemistry; Nuclear Structure; Nuclear Chemisty; Nuclear Reactions and Nuclear Energy; Radiation Chemistry; Nucleosynthesis of Elements; Chemical Periodicity of the Elements.

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Volume 2 Bonding Theories (VBT and MOT) of Covalency; Strucutre and Reactivity of Covalent Compounds; Stereochemical Nonrigidity and Fluxionality; Molecular Symmetry and Point Group; Solid State Chemistry - Structure and Bonding; Magnetic and Ferroelectric Materials.

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Volume 3 Bonding in Metals and Metal Clusters; PSPT - Wade's Rule and Jemmi's Rule; Electric Conductivities of Solids; Semiconductors and Superconductors; Hydrogen Bonding and other Weak Chemical Forces; Supramolecular Systems and Molecular Recognitions; Acids and Bases and Ionic Equilbria; Nonaqueous Solvents and Ionic Liquids; Redox Potentials, Formal Potentials and Applications; EMF Diagrams; Electroanalytical Techniques; Photoredox Reactions; Oscillating Reactions

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Volume 4 Complex Compounds - Introduction; Structure; Stereochemistry and Isomerism; Nomenclature, Bonding Theories (VST, CFT, LFT and MOT); Applications of CFT; J.T. Distortion; Spectrochemical Series; Stabilities of Complexes.

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Volume 5 Metal Complexes - Reaction Mechanisms (Ligand Substitution, Isomerisation, Racemisation, Electron Transfer and Photochemical Reactions); Electronic Spectra. Volume 6 Magnetochemistry and Magnetic Properties of Metal Complexes; Structure, Bonding and Reactivities of Organometallics including Metal Carbonyls and Nitrosyls; Organometallics as Catalysts.

Volume 7 Application of Metal Complexes in Analytical Chemistry and other Fields; Theory and Applications of Spectroscopic Methods (IR, Raman, NMR, ESR, Mossbauer, NQR, Mass Spectrometry, UV-VIS, UV-PES)

ra ry yl ib em ch al th e e/ t.m e

Mahua Das

MSc (CU), PhD (VisVQ Bharati)

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Former Research Associate, Department of Chemistry Visva Bharatt University, Santiniketan 731235 West BengaJ (tndia)

~ CBS

CBS Publishers & Distributors Pvt Ltd New Delhi • Bengaluru • Chennai • Kochi • Kolkata • Mumbai Hyderabad • Nagpur • Patna • Pune • Vijayawada

Disclaimer Science and technology are constantly changing fields. New research and experience broaden the scope of information and knowledge. The authors have tried their best in giving information available to them while preparing the material for this book. Although, all efforts have been made to ensure optimum accuracy of the material, yet it is quite possible some errors might have been left uncorrected. The publisher, the printer and the authors will not be held responsible for any inadvertent errors, omissions or inaccuracies. eISBN: 978-93-890-1756-4 Copyright © Authors and Publisher First eBook Edition: 2019

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All rights reserved. No part of this eBook may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system without permission, in writing, from the authors and the publisher.

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Fundamental Concepts of

Published by Satish Kumar Jain and produced by Varun Jain for Inorganic Chemistry

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CBS Publishers & Distributors Pvt. Ltd.

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Corporate Office: 204 FIE, Industrial Area, Patparganj, New Delhi-110092 ISBN: 978-81-239-2354-3 Ph: +91-11-49344934; Fax: +91-11-49344935; Website: www.cbspd.com; www.eduport-global.com; E-mail: [email protected]; [email protected] Copyright © Authors and Publisher

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Head Office:2014 CBS PLAZA, 4819/XI Prahlad Street, 24 Ansari Road, Daryaganj, New Delhi-110002, India. First Edition: Ph: +91-11-23289259, 23266861, 23266867; Fax: 011-23243014; Website: www.cbspd.com; Reprint: 2015, 2016 E-mail: [email protected]; [email protected]. All rights reserved. No part of this book may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system without permission, in writing, from the authors and the publisher.

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Branches

Published by Satish Kumar Jain and produced by Varun Jain for

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nd th Bengaluru: Seema House CBS Publishers Be Distributors Pvt2975, Ltd 17 Cross, K.R. Road, Banasankari 2 Stage, Bengaluru - 560070,

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4819/XI Kamataka Prahlad Street, Ansari Road, Daryaganj, Delhi 110002, India. Ph: 24 +91-80-26771678/79; Fax:New +91-80-26771680; E-mail: [email protected] Ph: 23289259, 23266861, 23266867 Website: www.cbspd.com Chennai: No.7, Subbarayae-mail: [email protected]; Shenoy Nagar Chennai - 600030, Tamil Nadu Fax: 011-23243014 [email protected]. Corporate Office: 204 FIE, Industrial Area, Patparganj, Deihl 110092

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Kochi: 36/14 Kalluvilakam, Lissie Hospital Road, Kochi - 682018, Kerala Branches Ph: +91-484-4059061-65; Fax: +91-484-4059065; E-mail: [email protected]

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Representatives Pune • Hydarabad 0-9885175004 • Nagpur • Puna Nagpur 0-9623451994· Vljayawada

0-9021734563· Patna 0-9334159340 0-9000660880

PrintedManipal ot India Binding House, Naida (UP)

Vijayawada Patna

A tribute to Prof Priyadara'1ian Ray

(1888-:1982)

who made a significant contribution chemistry

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in the field of

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present volumes 4-7 are in continuation of the existing title Fundamental Concepts Chemistry. The classnotes and valuable suggestions received from the esteemed readers have been shaped in these volumes.

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.1. of Inorganic

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These volumes cover the structure and bonding (VBT, CFf, and MOT), stability and reactivity, spectral and magnetic properties ofmetal complexes in depth. Kinetics and reaction mechanisms of ligand substitution, electron transfer and photochemical reactions have been included. Magnetochemistry and organometallic chemistry have been covered. Applications of different spectroscopic techniques (Raman, IR, NMR, ESR, Mossbauer, UV-VIS, UVPES, etc.) have been discussed to widen the utility of the series. In developing the present extension, we have taken all the measures to retain the basic features of the existing title.

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In preparing the manuscript, we have freely consulted the books and reviews of the earlier authors and have borrowed their ideas whenever it has been required. These sources are listed and acknowledged at the end of the text. We are grateful and indebted to these authors. In reality, we have picked up flowers from these gardens to prepare the garland to worship the goddess of learning. We are extremely thankful and grateful to Mr SK Jain, Managing Director, CBS Publishers

& Distributors, for his continued support. We are thankful to Mr YN Arjuna, Senior Director, Publishing, Editorial and Publicity, and the DTP staff for taking the trouble in processing the manuscript. In spite of our best efforts, some mistakes and misconceptions might have crept in for which we beg to be excused. Constructive criticism and suggestions are always welcome to better the presentation.

Asim K. Das Mahua Das

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11.1 Metal Complexes in Analytical Chemistry Inorganic Qualitative Analysis; Separation of Metal Ions Through Solvent Extraction; Complexation in Spectrophotometric Analysis; Complexation in Gravimetric Analysis; Masking or Sequestring Agents in Analytical Chemistry; Metal Complexes in Complexometric Titrations

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11.2 Metal Complexes in Medicinal Chemistry

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1681 1683

11.4 Metal Complexes in agriculture

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11.3 Metal complexes in industrial processes Organometallic Compounds as the Catalysts; Complexation in Water Softening; Complexation in Food Preservation; Complexation in the Rasching Process of Hydrazine Preparation; Complexation in Electroplating Process; Compl~xation in Metallurgical Process; Complexation a.nd Photograp~y

1687

11.6 Complexation and Stabilization of Unusual Oxidation States

1687

11.7 Complexometric Titrations Ethylenediaminetetraacetic Acid (called also Versene, Complexone-III): A Potential Complexometric Agent; A quantitative Treatment of Complexometric Titration; Effect of pH on the Complexometric Titration Curves for Ethylenediaminetetraacetic Acid as the Complexing Agent; Minimum Required Value of C~nditional or Effective Stability Constant and Suitable pH Range for Titration by Ethylenediaminetetraacetic Acid; Effect of Other Complexing Agents on Complexometric Titration Curves; Effect of other Complexing Ligands on the Conditional Stability Constant; Metal Ion Buffer; Metal Ion or Metallochromic Indicators; Selection of Metal Ion Indicator in Complexometric Titration: A Quantitative Treatment; Different Types of Complexometric Titrations; Selectivity in Titration with edta by Controlling pH

1687

11.8 Mercurimetric Titration of Halides: Formation of Multiple Complexes during Titration

1713

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11.5 Metal Complexes and Solar Energy Conversion

Exercise 11

1714

Appendix-lIA: Distribution of Different Species of EDTA in Different pH Values

1718

Fundamental Concepts of Inorganic Chemistry

1719-2111 12.1 Vibrational Spectroscopy: Infra-Red (IR) and Raman Spectra

1719

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Some Important Aspects of the IR and Raman Spectra; Classification of Normal Modes of Molecular Vibration; Normal Modes of Vibration (See Appendix 12B) and Infrared and Raman Active Modes in Some Simple Molecules; Characteristic Group Vibrations; Electronic and Coupling Effect on Group Vibration Frequency; Limitations of the Characteristic Group Vibrations; Application of Infrared and Raman Spectra: Determination of Structure of Some Simple Molecules; Symmetry of the Normal Modes of Vibration (3n - 6 or 3n - 5) and Number of ir-Active and Raman Modes (See Appendix-12B); Application of Infrared and Raman Spectra: Determination of Structure through the Normal Mode Analysis; Application of Infrared and Raman Spectra: Determination of Geometrical Isomers of Coordination Compound~; Application of Infrared and Raman Spectra: Determination of the Structures of Metal Carbonlys; Effect of Coordination on the Rotational-Vibrational Spectra of the Ligands; Application of Infrared Spectra: Appearance of New Bond Vibration for the Metal-Ligand Bond Formation; Application of Infrared and Raman Spectra: Appearance of New Bands of the Ligand upon Coordination; Application of Infrared Spectrum: Detection of Water in Coordination Compounds; Application of Infrared Spectra: Shifting of Ligand Band Position to Characterise the Mode of Coordination; Application of Infrared Spectra: Identification of Linkage Isomers; Application of Infrared and Raman Spectra: Assignment of the Coordinating Behaviour of the Anionic Ligands (XOr, XO,r)

12.2 NMR (Nuclear Magnetic Resonance) Spectroscopy and Its Application

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Some Basic Elements of NMR Spectroscopy; Intensity of the NMR Signals and Peak Broadening; Relaxation Processes: Sharp Peak vs. Intensity of the Peak; Concept of Chemical Shifts and Solvents for PMR Studies; Factors Controlling the Magnitude and Direction of Chemical Shift; Chemical Shifts for the PMR Signals in Metal Hydrides and Organometallics; Area of the NMR Peak and Number of Nuclei: Integration Principle; Spin-Spin Coupling and Fine Structure of the NMR Signals; Spin-Spin Coupling and Coupling Constant (J); Magnetically Equivalent and Nonequivalent Nuclei: Magnetic Equivalence vs. Chemical Equivalence; Designation (i.e. Nomenclature) of Nuclei Spin System; First Order and Second Order NMR Spectra: Advantage of the NMR Spectrometer Operating at Higher Frequency (e.g. 60 MHz vs 100 or 300 MHz); Chemical Shift Reagents; Nuclear Magnetic Double Resonance (NMDR) and Spin Decoupling - A Way to Simplify the Complex NMR Spectra: Noise Decoupling ~nd Nuclear Overhauser Effect (NOE); Effect of Chemical Exchange on NMR Spectra and Evaluation of Exchange Rate Constants; Quadrupole Effects and NMR Studies for the Nuclei with Spin Greater than 1/2: Quadruple Peak Broadening and Decoupling; 13C-NMR Spectroscopy and Organometallics; NMR Spectroscopy for the Nuclei other than Hydrogen: PMR Spectroscopy vs. Other NMR Spectroscopy; Application of NMR Spectroscopy in Structure Determination: Some Representative Examples; Application of NMR Spectroscopy in Identification of Rotational Isomerism; Application of NMR Spectroscopy in Detection of H-bonding and in Distinction of Intramolecular and Intermolecular H-bonding; Application of NMR Spectroscopy to Determine the Relative Amounts of the Keto-Enol Tautomers in a Tautomeric Mixture; Application of NMR Spectroscopy for Determination of Exchange Rate Constant; Application of NMR Spectroscopy and MRI (Magnetic Resonance Imaging); Fluxionality through the Intramolecular Rearrangement and NMR Studies; Two-Dimensional NMR - ROSY, COSY, NOESY

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Contents xi 1870

12.4 MoSsbauer (MB) Spectroscopy Isomeric Nuclides (of an Element) Differing in Energy States: Nuclear Transition Energy in the y-Region; Principle of Mossbauer (MB) Spectroscopy: Condition of Recoilless Emission and Absorption of y-Ray through the Adjustment of Doppler Shift; Source and Absorber: MB Spectroscopy for 57Fe and 119Sn; Isomer Shift or Centre Shift or Chemical Isomer Shift (in short, Chemical Shift) in MB Spectroscopy; Quadrupole Interaction and Splitting of the MB Spectral Lines; Effect of a Magnetic Field on the MB Spectrum: Magnetic Hyperfine Interaction; Application of MB Spectroscopy

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12.3 Electron Spin Resonance (ESR) Spectroscopy Interaction between the Electron Spin and Magnetic Field: Some Basic Elements of ESR Spectroscopy;' Relaxation Processes: Intensity and Bandwidths of the ESR Signals; Fine Structure of ESR Signals: Zero-field (Crystal Field) Splitting: Kramers' Degeneracy; Zerofield Splitting and Effective Spin (S') and Effective ESR Transitions in Different tf Systems of the 1st Transition Series; Nuclear Spin and Hyperfine Splitting of the ESR Signals; Mechanism of Coupling of the Electronic Spin and Nuclear Spin; Illustration of the Fine Structure and Hyperfine Structure of the ESR Signals; ESR Spectra: Magnetically Concentrated vs. Magnetically Diluted Complexes; Factors Affecting the g-value; Determination of the gvalue; Anisotropic Behaviour of g and Anisotropy in Hyperfine Interaction; Anisotropic Behaviour of the g-value of the Tetragonally Distorted Copper(ll) Complexes; Anisotropic Behaviour of the g- Value for the Tetragonally Distorted Nickel(II) Complexes; EPR Peak Broadening and Peak Merging and Electron Spin Exchange Rate; Applications of ESR Spectroscopy

1986

12.6 Electronic States of Diatomic Molecules and Electronic Spectra of Diatomic Molecules Electronic States and Term Symbols of the Diatomic Molecules and Ions; Selection Rules of Electronic Transitions in the Diatomic Molecules: Electronic Spectrum of Molecular Hydrogen and Oxygen

2021

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12.5 Electronic Spectroscopy: UV-Visible Spectroscopy Electronic Spectroscopy of the Transition Metal Complexes; Electronic Transitions in Organic Molecules; Franck-Condon Principle and the Selection Rule; Designation of the UV-Bands of Electronic Absorption; Important Terms Involved in the Electronic Spectra: Chromophores and Auxochromes; Bathochromic Shift (i.e. Red Shift) and Hypsochromic Shift (Blue Shift); Hypochromic and Hyperchromic Shift; Isosbestic Point; Solvent Cutoff Points; Effect of Resonance and conjugation on the Position and Intensity of the Absorption Bands; Solvent Effect (i.e. Solvatochromism or Solvatochromic Shift) on the Position of the Absorption Bands; Effect of Intramolecular H-Bonding on the Positions of the Absorption Bands: Identification of Keto-Enol Tautomerism; Characteristics of the Absorption Bands in Alkenes and Polyenes; Characteristics of the Absorption Bands of the Carbonyl Compounds; Characteristics of the Absorption Bands in the Aromatic Compounds; Colour of Halogens and Different Colours of Iodine (12) and Bromine (Br2) in Different Solvents 2020

12.7 Ultraviolet Photoelectron Spectroscopy (UV-PES) and Identification of the Nature of Molecular Orbital Energy Level 12.8 Nuclear Quadrupole Resonance (NQR) Spectroscopy and Its Application Quadrupole Nucleus, Nuclear Quadrupole Moment, Electric Field Gradient (EFG) and Asymmetry Parameter (11) ; Principles of.NQR Spectroscopy: Quadrupole Energy States and Interaction with the Electromagnetic Radiation; NQR Transition Energies for the Axially Symmetric Systems (i.e. 11 = 0); NQR Transition Energies for the Nonaxially Symmetric Systems (i.e. 11 i:- 0); Effect of a Magnetic Field (Zeeman Effect) on NQR Transitions;

2031 2031

xii

Fundamental Concepts of Inorganic Chemistry Conditions (Summary) to observe the NQR Signals; Solid State Effect and the Magnitude of Nuclear Quadrupolar Coupling Constant Determined from the NQR Studies; Application of NQR Spectroscopy

12.9 Mass Spectrometry and Its Application Working Principle of a Mass Spectrometer; Ionisation and Fragmentation of the Sample: Fragmentation Mode-a Fingerprint of the Sample; Application of Mass Spectrometry

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2069

12.11 Magnetic Moment and Application of the Knowledge of Magnetic Moment Data

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12.12 Some Important Electroanalytical Techniques Like Potentiometry, Coulometry, Amperometry, Polarography and Cyclic Voltammetry (CV)

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12.13 Other Instrumental Methods of Chemical Analysis

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12.10 Optical Rotatory Dispersion (ORD) and Circular Dichorism (CD) and Application of ORD and CD Curves to Determine the Absolute Configuration

Exercise 12

2078 2095

Appendix 12B: Normal Vibrational Modes for Some Common Molecular Structures

2107

Appendix 12C: Splitting of ground state T-term (octahedral and tetrahedral Fields) by Spin-Orbit Coupling and Magnetic Field (i.e. Zeeman Effect)

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2112-2192

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Appendix 12A: Character Tables of Different Point Groups (Excluding the Cubic Functions)

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13.2 Important Terms in Error Analysis

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13.1 Introduction to Errors in Analysis

2118

13.4 Characteristics of the Random or Indeterminate Errors

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13.3 Characteristics of Systematic Errors

2121

13.6 Population Mean (m), Sample Arithmatic Mean (X), and Measures of DispersionRange, Populatiola Standard Deviation (s) and Sample Standard Deviation (s)

2126

13.7 Confidence Interval (CI) and Confidence Level (CL) and Probable Random Analytical Error

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13.5 Distribution of Random Errors: Statistical Treatment of Random Errors

13.8 Testing for Significance (Hypothesis Testing) and Criteria for Rejection of an Observation Comparison of the Mean Values (t-test); Comparison of Precision (Variance Ratio Test or F-test) ; Criteria for Rejection of an Observation 13.9 Propagation of Random Error: Accumulation of Random Error in the Compound Quantities 13.10 Rounding-off Numbers, Significant Figures and Computation Rules Rounding-off Numbers and Rules of Rounding-off Numerical Values; Concept of Significant Figures; Significant Figures in Numerical Computation of Addition and Subtraction;

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2143 2153

Contents

xiii

Significant Figures in Numerical Computation of Multiplication and Division; Significant Figures in Numerical Computation of Logarithms and Antilogarithms

13.11 Correlation Coefficient, Regression Analysis of Straight Line Relationship (i.e. Method of Least Squares) and Regression Coefficient

2162

Correlation Analysis and Correlation Coefficient; Linear Regression Equation: Least Squares Method

2189

Numerical Problems

2191

Bibliography

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Exercise 13

Appendix

I: Units and Conversion Factors

Appendix II: Some Physical and Chemical Constants

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Appendix III: Wavelength and Colours

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Appendices

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Appendix IV: Names, Symbols, Atomic Numbers and Atomic Weights of the Elements

A-1-A-9 A-I A-3 A-4 A-5 A-6

Appendix VI: Books Consulted

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Appendix V: Some Useful Mathematical Relationships

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Subject Index

1-1-1-4

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Application of Complex Compounds

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11.1 METAL COMPLEXES IN ANALYTICAL CHEMISTRY

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11.1.1 Inorganic Qualitative Analysis

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(A) Separation of AgCI from Hg2 CI 2 and PbCI2 in Gr. 1 of qualitative analysis: From the Group 1 precipitate containing AgCI, Hg2Cl 2 and PbCI2, the components are separated as follows: Gr.1 precipitate is boiled with water and filtered when hot

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Filtrate contains PbCI 2 (PbCI 2 is soluble in hot water but not in cold water)

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Residue (H9 2 C1 2 + AgCI)

H

Filtrate [Ag(NH 3)2l +

(Ammine complex)

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Reactions:

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Residue (black) (Hg + Hg(NH 2 )CI)

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Treatment with dil. NH 3

• Hg 2Cl 2 + 2NH 3 • AgCI + 2NH 3

Disproportionation of Hg~+

I

-------~) Hg~ Complexation)

+ Hg(NH 2)CI

I

~

+ NH4+ + CI-

[Ag(NH 3)2]+ + Cl-

Thus AgCI is separated through the ammine complex formation (B) Separation of Gr-IIA and lIB sulfides in qualitative analysis: When the mixture of precipitate is treated with yellow ammonium sulfide, the Gr-IIB sulfides (i.e. sulfides of arsenic, antimony and tin) produce the complex thiosalts which are soluble. 1659

1660

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

Gr-IIA sulfides (i.e. HgS, PbS, Bi 2S3 , CuS, CdS) + Gr-IIB sulfides (i.e. AS2S3 + Sb2S3 + SnS) yellow ammonium sulfide i.e. (NH 4)2 Sx solution

Residue (Gr-IIA sulfides)

Filtrate (Thiocomplex of Gr-IIB precipitate)

SnS + (NH4)2Sx ~ 2NHt + SnS1- + (x - 2)S

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As 2S S + 3(NH4)2Sx ~ 6NHt + 2AsSl- + (3x - 3)S ~ Sb 2S 3 + 3(NH4)2Sx ~ 6NHt + 2SbSl- + (3x - 5)S ~

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AS 2S 3 + 3(NH4)2Sx ~ 6NHt + 2AsSl- + (3x - 5)S ~

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The Gr-lIB sulfides are solubilized through the oxidation followed by the formation of thiocomplexes like Assl- (thioarsenate), SbSl- and SnS1-. The Gr-IIA sulfides fail to form the complexes under this condition. (Note: Yellow ammonium sulfide i.e. (NH4)2Sx possesses the oxidizing action (e.g. AS IlI ~ As v ; Sn Il ~ Sn IV ) probably due to the presence of some sulfur in zero oxidation state. Thus, (NH4)2Sx may be considered as (NH4)2S + (x - I)S. Thus, the simple ammonium sulfide i.e. (NH4)2S does not have any oxidizing action). The thiocomplexes of Gr. lIB sulfides are easily decomposed by acidification with HCI.

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2Assl- + 6H+ ~ As 2S S ~ + 3H2S~ As 2S S ~ AS 2S 3 ~ + 2S ~ 2SbSl- + 6H+ ~ Sb2Ss ~ + 3H2S; SnS1- + 2H+ ~ SnS2 J,. + H 2S.

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(C) Separation of Cd2+ and Cu2+ in Gr. IIA analysis: In an acidic condition, both Cd2+ and Cu 2+ are

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precipitated as sulfides. They are separated as follows: 1 : 1 HN03

2

2

dil. NH 3

2

~

L-y---J Solution

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CuS + CdS - - -... ~ Cu ++ Cd + - -... ~ [Cu(NH 3)4]2+ + [Cd(NH 3)4] +; (ammine complexes)

Precipitate of CdS

(i)KCN (ii) H2S

Filtrate of [Cu(CN)i~"

In presence of excess CN-, Cu(ll) is reduced and complexed as follows: 2Cu 2+ +4CN- ~2Cu(CN)2~2CuCN+(CN)2 i (unstable)

2CuCN + 6CN- ~ 2[Cu(CN)4]3- (cyanido complex). Formation of stable [CU(CN)4]3- raises the formal reduction potential of the Cu(II)/Cu(l) couple to allow the oxidation of (CN)- to (CN)2 (i.e. cyanogen gas) Though the solubility product of CdS is much higher than that of CU2S, in presence of H 2S (in ammoniacal medium), [Cd(CN)J 2- decomposes to give the precipitate of CdS. It happens so due to the

1661

ApPLICATION OF COMPLEX COMPOUNDS

higher stability of [CU(CN)4]3- where CN- is a Tt-acid ligand. The metal ~ ligand Tt-back bonding is favoured for the lower oxidation state of metal centre. It makes the complex [CU(CN)4]3- more stable. The detailed chemistry behind the separation of Cu(II) and Cd(II) has been discussed in Sees. 14.17.3 and 16.4.4 of Vol. 3. (Note: Separation of Cu 2+ and Cd2+ through complexation as stated above is an example of masking).

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(D) Identification of metal ions by following the characteristic colour of their complexes: Very often, the complexes possess the characteristic colour. The coloured complex may be soluble or insoluble. These complex formation reactions are used to identify the metal ions in qualitative analysis. In most of the cases, the characteristic intense colour originates from the charge transfer (CT) transitions. For the J> systems (e.g. Bell, Mg II , Pbll , CrVI , AIIII , Ti lv , etc.) and metal ions of high charge to radius ratio (e.g. FellI, COllI, etc.), the ligand to metal CT (i.e. LMCT) bands give the intense colour. For the lower valent metal ions (e.g. Fell), the intense colour may arise from the MLCT bands. However, in some cases, (e.g. tetrahedral, square planar complexes), the colour may appear due to the d-d transitions. Some representative examples are given below.

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(i) NH3 solution: [Cu(NH3)4]2+ (deep blue solution (d-d transition)): (ii) Salicylaldoxime (LH) (in acetic acid solution):

-)Q> dO~- ;:c( _

H

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C==NOH

~O y-

" C==N

OH + Cu 2+

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O----H

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\

:

N-C,

I

+

+2H

H

H--O [CU(L)2l

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(LH 2)

0I

(Greenish yellow precipitate)

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bisalicylaldoximatocopper(ll)

C

(Note: Pd(II) gives a similar precipitate in acetic acid solution) (iii) Benzoinoxime (cupron) : In neutral or ammoniacal solution, Cu(II) produces a green complex that precipitates (probably in a polymeric form involving the N-O-Cu bridge). Ph

Ph

'"I

'"I

CH-O

CH-OH +

CUll -

C===N-OH

Ph/

(iv) Rubeanic acid (i.e. dithiooxamide):

,>CU II +2H+

C===N

Ph/

' "o

1662

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

NH

C~

S Cu

2 +

+ 2LH 2 ~ Cu

I

S-----C~

~NH

or,

Cu

2+

+ LH 2 ~ CuL + 2H

+

em

yl ib

ra ry

In fact, in weakly acidic or ammoniacal solution, Cu2+ gives a polymeric and extremely insoluble black coloured complex. This reagent is the most sensitive reagent known for the detection of Cu 2+. It can detect Cu 2+ at a concentration of 1 ppb. This method for detection of Cu2+ was developed by Prof. P. Ray, an Indian chemist. (Note: Under the similar condition, Co(ll) gives a brown insoluble complex and Ni(ll) gives a blue to green insoluble complex). Pd(ll) and Pt(ll) also form the coloured complex with the semi-acid form of the ligand. H2

N--C=S

=Pd, Pt

ch M

.

NH2.

z.e.

tu

e

/ (i) Thiourea: S=C\.

t.m

e/

th e

I S--C=NH

al

M

er

NH 2

C

lic

k

H

In dilute nitric acid solution, Bi(lll) forms a yellow coloured (LMCT) soluble complex, [Bi(SC(NH 2)2)3]3+ i.e. [Bi(tu)3]3+. (ii) KI solution: In acidic solution, Bi(lll) gives the black precipitate of BiI 3 which dissolves in presence of excess KI to give an orange coloured solution. Bi 3+ + 31- ~ Bil 3 (J,);BiI 3 (s) + 1- ~[BiI4J (orange) cf. Hg 2+ + 21- ~ Hgl 2 (red) J,; Hgl 2 (s) + 21- ~[HgI4j- (colourless).

I Fe2+

and Fe3+

I

(i) 2, 2'-bipyridine (bpy): Fe2+ion produces the deep red [Fe(bpY)1J 2+complex (MLCT band: metal to vacant 1[* -MO of ligand). (ii) o-phenanthroline (phen): In the reaction with phen, Fe 2+ also produces the deep red coloured [Fe(phen)3]2+ complex (called ferroin). Note: Fe 3+ ion produces the very light blue coloured complexes with the bpy or phen ligands. In dilute solution, they appear almost colourless. If Fe3+ is reduced by NH 20H, HCI (i.e. hydroxyl amine hydrochloride) to Fe 2+ then the reduced Fe2+ can be detected by using the bpy or phen ligand.

1663

ApPLICATION OF COMPLEX COMPOUNDS

yl ib

ra ry

[Fe(L-L)3]3+ (almost colourless) Reduction) [Fe(L-L)3]2+ (deep red coloured) (L-L = bpy or phen) (i) SCN- (thiocyanate): Fe3+ gives the red coloured complex [Fe(SCN)x ](x-3)-. This colour is due to ligand to metal charge transfer (LMCT). Nc:'f: • Organic acids like oxalic acid, tartaric acid, etc form the colourless complexes with Fe3+. These complexes, e.g. [Fe(C 20 4)3 ]3-, [Fe(C4H60 2)3 ]3-, etc. are colourless and such organic chelating ligands (having the O-donors) interfere with the complexation by SCN-. • Fluorides can discharge the red colour of [Fe(SCN)x](x-3)- due to replacement of SCN- by P- leading to the colourless [FeF613- complex. In fact, Fe(III) being hard prefers P- compared to SCN-. • Hg2+ being softer than Fe3+ can snatch away the Fe(III)-bound soft SCN-'ligand to discharge the red colour. In fact, Hg(ll) forms the more stable [Hg(SCN)4]2- complex. (ii) Ferron or 7-iodo-8-hydroxyquinoline-5-sulfonic acid:

ch

em

S03H

al

Fe

th e

HO

t.m

e/

The tris-chelate is green (LMCT) coloured. (Note: Fe2+ does not react with ferroin under the condition). (iii) With tiron (i.e. disodium 1,2 dihydroxybenzene-3,5-disulfonate).

~ Fe

0

~.~

) F e 111/3 + 2H+

H

o SO;

lic

k

so;

-03S~0

3+ _ _

er

IOH

+

e

-OaS1 6 ( 0H

(Red complex, LMCT)

C

Note: 1: 1 Complex - blue; 1:2 Complex - violet; 1:3 Complex - red. (iv) With hydroxamic acid, Fe(lll) also gives the characteristic colour (LMCT). R-C=O

I

""Fell~3

HN-O/

(v) Cupferron (i.e. ammonium salt of nitrosophenylhydroxylamine)

@-j-O--NH; N=O

Fe

(Cupferron)

In aqueous HCI solution, Fe 3+ gives a reddish-brown (LMCT) precipitate of the complex, [Fe{C6HsN(NO)O}3l

1664

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

ra ry

Note: The ligand was initially used for detection of iron and copper and this is why the ligand was named so. (vi) Sulfosalicylic acid: Fe(II) does not give any colouration but Fe(III) gives the violet colouration.

em

yl ib

(vii) DmgH2 : In ammoniacal solution, Fe(II) gives the red coloured complex [Fe(dmgH)3]- but Fe(III) does not respond to this test (cf NiH + dmgH 2). \

o

ch

(i) Alizarin and Alizarin-S:

o

OH

al

OH

OH

o

o

(Alizarin-S i.e. sodium salt of alizarin sulfonate)

er

e

(Alizarin)

t.m

e/

th e

OH

AI 111/3

/A\,H)x(OH 2)6-2_X

H

/ \0

o

k

o

0

OH

C

lic

OH

o AI-Complex 1:3

o AI-Alizarin Complex 1: 1

In a slightly alkaline condi~ion, A1 3+ forms a slightly soluble red coloured (LMCT) complex (called aluminium lake) with the titled ligands. Note: Zirconium(IV) (i.e. Zr02+, zirconyl cation) also forms a similar complex of red-violet colour and the complex is more stable than the AI(III)-complex. In fact, the Zr(IV)-complex is stable even in a strongly acidic condition while the corresponding AI(III)-complex is decomposed in an acidic medium. However, the Zr(IV)-complex is decomposed by P- giving rise to the more stable [ZrF6 ]2- complex (cf. hard-hard matching) which is colourless. (ii) Aluminon: This is the ammonium salt of aurine tricarboxylic acid. It gives a red coloured (LMCT) lake of aluminium in a weakly alkaline solution of AI(III)

1665

ApPLICATION OF COMPLEX COMPOUNDS

HO

o

HO

ra ry

CO;NH;

yl ib

(Aluminon)

ch

C

al

o ~

e/

th e

HO

em

CO;NH;

CO;NH;

t.m

HO

e

AI(III)-Complex (1 : 1 Complex)

lic

k

H

er

It may form also the tris-complex i.e. 1:3 complex

C

(i) NH4SCN: C0 2+ forms the blue coloured (d-d transition; high E-value in the tetrahedral complex)

complex, [CO(SCN)4]2- which can be extracted in amyl alcohol layer as H 2[Co(SCN)4] (Note: Interference by Fe3+ can be removed by using the masking reagent P-). (ii) KN02 solution: C0 2+ gives the yellow precipitate of K3 [Co(N02)6].3H20. The reaction is carried out in a weakly acidic condition (say acetic acid medium). NOi oxidizes C02+ to C03+ and NOi is itself reduced to NO. Then C0 3+ undergoes complexation with the excess ligand NOi. Co 2+

N~~~~;

) Co 3+

excess KN0

2

)

K 3 [ Co (N0 2 )6 ]

(iii) Rubeanic acid: In an ammoniacal medium, with the ligand, Co(II) forms a yellow-brown complex which is insoluble. (iv) a-Nitroso-p-naphthol: It remains in a tautomeric equilibrium. Its tautomeric oxime form makes an inner complex of Co(III). The reagent can itself act as an oxidizing agent to oxidise Co(Il) to Co(IIl) because it is an orthoquinoid compound and a derivative of HN0 2• But it is convenient to oxidise Co(II) to Co(III) before applying the reagent.

1666

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

0~N--COIll/3

NO

I

o

OH

co3+ ---. -H+

Of,

(Note: In presence of the title chelating ligand, oxidation of Co(ll) to Co(lll) by air is also possible).

ra ry

I VV i.e. Vanadate I

)2(S04)3

th e

al

~.,~ (v 100)

[ !(~)]l/2 P 100+100

0.1(100 - VL ) 100+ VL

100+ L ) 1+log -V ( 100- V L

1 1 200 -logP+-Iog2 2 10 1 ="2logP + 0.65

Note: -log [M] = pM; actually conditional or effective stability constant stability constant (~) in all these calculations.

1

0.lxl00

P

O.I(VL -l00)

-x---logP - 2 + log(VL -100)

(~')

is to be used instead of thermodynamic

1692

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

Thus it is evident that before and after the equivalence point, pM increases with the increase of VL (i.e. volume of titre) and at the equivalence point it is only dependent on p. The degree of sudden change of pM value at the equivalence point depends on the magnitude of the stability constant. In fact, the plot of pM vs. VL (i.e. titration curve) is very much comparable with that of the acid-base titration curve (i.e. pH vs. V). It is illustrated in Fig. 11.7.2.1 by taking some representative examples. Titration curve (i.e. pM vs. VL ) can be constructed as shown in Table 11.7.2.4. Table 11.7.2.4 Construction of a titration curve in a representative complexometric titration (say log~ = 10.0) T~ = T~ = 0.1 moldm-3 , V~ = 100 mL

P= 10.0

ra ry

log

10

50

90

99

99.9

100

100.1

110

1.1

1.48

2.28

3.3

4.3

5.65

7.0

9.0

_

(100 + V

pM=~log~ 2 +0.65

em

1.0

yl ib

VL (mL): 0 pM:

L )

pM -1 + log 100 - V

th e

al

ch

L

pM = log~-2 +log(VL -100)

~(3)

er

e

t.m

e/

~(4) > ~(3) > ~(2) > ~(1)

lic

k

H

~(2)

C

~(1)

Equivalence point

~

Volume of edta solution

Fig. 11.7.2.1 Qualitative representation of the change of pM value at the equivalence point depending on the magnitude of the stability constant (~) of the metal-edta complex.

Validity of the titration depends on the sharp change of pM values at the region of equivalence point and this change depends on the stability constant ~ as evident from Table 11.7.2.5 and Fig. 11.7.2.1.

1693

ApPLICATION OF COMPLEX COMPOUNDS

Table 11.7.2.5 Change of pM value at the region of equivalence point in the complexometric titrations depending on the magnitude of the stability constants (B). T~ = T2 = O.IM, v~ = 100 mL

fJ

log

pM-value Equivalence point (VL = 100 mL)

pM-value Just before the equivalence point, VL = 99.9 mL (say) i.e. 0.1% under titration)

pM-value Just after the equivalence point VL = 100.1 mL (say) i.e. 0.1% over titration)

ApM

in the region of equivalence point

4.30

4.65

5.0

5.0 - 4.3 = 0.7

4.30

5.65

7.0

7.0 - 4.3

4.30

6.65

9.0

4.30

7.65

11.0

16

4.30

8.65

13.0

yl ib

12 14

ra ry

8 10

= 2.7

9.0 - 4.3 = 4.7

11.0 - 4.3 13.0 - 4.3

= 6.7 = 8.7

ch

em

11.7.3 Effect of pH on the ComplexometricTitration Curves for Ethylenediaminetetraacetic Acid as the Complexing Agent

al

Ethylenediaminetetraacetic acid (H 4edta) is commonly used in complexometric titrations. Stability constant (~) is expressed as follows:

th e

M + edta 4 - ~ [M(edta)], (charges not shown for the sake of simplicity)

e/

(Complex)

t.m

~ = [Complex] [M] [edta 4 - ]

H

er

e

In addition to the above equilibrium, there are number of proton-ligand competitive equilibria.

C

lic

k

Note: Here it is assumed that the first two strongly acidic protons of H 6 edta 2 + are almost completely lost in the working range of pH, . eXIstence . l.e. 0 fH 6ed ta 2+

+

3-

Hedta

T=':

4-

H + edta

, K a (4)

[H+ ] [edta 4 = [Hedta 3-]

and Hsedta + has been ignored (cf. Appendix ItA) ]

= 2.0, pKa = 2.7, pKa = 6.2, pKa = 10.3 at (20

0

C) Distribution of different protonated species of edta is shown in Fig. 11.7.3.1. It is evident that both the metal ion (M, charge not shown) and H+ mutually compete for edta4-. Under this condition, the conditional or effective stability constant (~/) is more important than the thermodynamic stability constant (J3) to understand the titration curve of complexometric titration. pKa(1)

(2)

(3)

(4)

1694

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

edta4-

Note: In reality, at pH < 2.0, besides the species H4edta and H3edta-, the species

ra ry

Hsedta+ and H6edta2+ also exist but these are ignored here. In fact, at pH ~ 0, Hsedta

2 +

is the most predominant species (see

6

-----.

8

10

pH

12

al

4

2

th e

o

ch

em

yl ib

Appendix 11A).

TM = [M] + [M(edta)]

+ [H 4edta] + [H 3edta-] + [Hzedta z-] + [Hedta 3-] + [edta 4-]

e

TL = [M(edta)]

t.m

From the mass-balance, we can write:

e/

Fig. 11.7.3.1 Distribution of different protonated species of edta (ignoring the H 6 edta2+ and H s edta+ species).

[M(edta)] =

[H+ ]3

H

{ K a(I)Ka(2)Ka(3)Ka(4)

+ K a(2)Ka(3)Ka(4)

C

lic

k

-

er

[H+ ]4

TL

=(XL [edta 4 - ], (XL or,

depends on pH).

[edta 4-] = TL -[M(edta)] (XL

K a(4), K a(3), ••• are the successive deprotonation constants of H 4edta. Thus for a particular ligand, (XL is a function of pH (cf. Table 11.7.3.1 and Fig. 11.7.3.2).

1695

o

th e

al

ch

em

yl ib

ra ry

ApPLICATION OF COMPLEX COMPOUNDS

4

6

8

10

t.m

e/

2

Fig. 11.7.3.2 Variation of log u L as a function of pH for edta.

can be expressed in terms of aL-

H

~ = [M(edta)]

(~)

er

e

Thus the thermodynamic stability constant

C

lic

k

[M] [edta 4- ]

{TM

-

=

[M(edta)]a L

[M(edta) ]}{TL

~' a v (~'

.

.

[M(edta) ]}

= conditional or effective stability constant)

i.e.

log ~

= log ~' + log a L

or,

log ~'

= log

~

-

..

; (Ignonng the hydrolysIs of the metal Ion).

- log aL

From the known Ka-values of H 4edta, the parameter a L (which is a function of pH) can be calculated (cf Table 11.7.3.1). The plot of 10g(aL) vs. pH is shown in Fig. 11.7.3.2.

Note: If the metal undergoes complexation with the other ligands including OH- (i.e. hydrolysis of the metal centre), then we have: ~ = ~'a~L i.e. log ~ = log ~' + log aL + log aM, (cf Sec. 11.7.5 and Chapter 4). Here for the sake of simplicity, aM is taken as unity (i.e. no complexation with the other ligands).

1696

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

Table 11.7.3.1 Values of UL for edta at different values uL 10 18 10 13 lOll 10 10 108 106 104 103 102

18.0 13.44 11.86 10.60 8.43 6.46 4.65 3.33 2.27 1.28 0.46 0.07 -0.00

em

1X 2.75 x 7.24 x 3.98 x 2.75 x 2.9 x 4.46 x 2.13 x 1.86 x 19.0 2.88 1.18 1.02

uL

ra ry

1.0 2.0 2.5 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0 12.0

log

yl ib

pH

al

ch

11.7.4 Minimum Required Value of Conditional or Effective Stability Constant and Suitable pH Range for Titration by Ethylenediaminetetraacetic Acid

t.m

e/

th e

For a successful complexometric titration (i.e. sufficient and sudden change of the pM value at the equivalence point in the titration curve), conditional stability constant CB') should be atleast 108 i. e. log p' ~ 8.0 (ef Table 11.7.2.5, L\pM ::::: 0.7 for log~' = 8) Let us consider the titration of Ca2+ by edta for which log B= 10.7, i.e.

B= log B' + log u L log u L = log B- log B' = 10.7 - 8.0 = 2.7 log

er

e

or,

H

pH = 12

C

lic

k

pH = 10

pH =8 ct3

U

Q.

r ----.~

Vol. of edta solution

Fig. 11.7.4.1 Qualitative representation of the effect of the pH on the nature of titration curve for titration of Ca2+ by edta solution.

1697

ApPLICATION OF COMPLEX COMPOUNDS

[M(edta)] + nOH- ~ M(OH)n j, + edta4 -

ra ry

From Fig. 11.7.3.2, it is evident the required a L (i.e. log a L = 2.7) is attained at pH = 7-8. In practice, the maintained pH is around 10.0. At lower pH, the conditional stability constant is less and consequently the change in pCa in the range of equivalence point is less. It makes the titration erratic. For Pb 2+, log ~ = 18.0, the required condition is: log a L = log ~ - log ~' = 18.0 - 8.0 = 10.0 This required a L is attained at much lower pH. Thus it may be concluded that, with the increase of thermodynamic stability constant (P), the minimum required value of conditional stability constant (log ~' ~ 8.0) is attained at a lower pH. Note: At higher pH (i.e. higher concentration of OH-), participation of OH- as ligands may complicate the situation. It may lead to the hydroxo-species.

yl ib

Thus to maintain the required condition (i.e. log ~' ~ 8), pH cannot be increased arbitrarily. This is why, for the systems having very low stability constants, the complexometric titration cannot be done.

em

Illustration 1: 100 mL of 0.01 M Ca2+ solution is being titrated with 0.01 M edta solution at the buffered pH 10. Calculate the change of pCa at the equivalence point.

=5 X

1010,

a edta

= 2.85 (at pH = 10),

ch

~[Ca(edta)]

Given:

al

a Ca = 1 (i.e. no complexing ligand other than edta).

e.

A'

fJ [Ca(edta)]

5 X 10 ~[Ca(edta)] - - = 1.75 X 1010. (a = 1) 2.85 x l ' Ca 10

aCaaedta

= 1 indicates that the metal ions not bound to edta remain as free ion, i.e.

TM - [M(edta)]

e

aCa

(d"· con thanaI stab·l" t tty constant ) =

t.m

.

I.

= ~'[Ca(edta)]aCaaedta

e/

~[Ca(edta)]

th e

Solution: We have the relationship.

er

pCa can be calculated at the different stages of titration as follows:

= -log[Ca2+] = 2.0

lic

k

H

(i) At the start of titration (i.e. VL = 0): [Ca2+] = 0.01 M i.e. pCa (ii) After the addition 30 mL of edta:

C

[Ca2+) = 0.01 (100 - 30) M = 5.38 X 10-3 M 100+30 i.e. pCa = 2.26 (iii) After the addition 99.9 mL of edta (i.e. just before the equivalent point): 99.9) M = 5 X 10-6 M 100 + 99.9 i.e. pCa = 5.30 (iv) At the equivalent point (i.e. VL = 100 mL) [Ca2+)

= 0.01 (100 -

At this point, [Ca(edta)] z 100 x 0.01 M 200

~'

_

[M(edta)] -

=5 X

10-3 M

[M(edta)] [Cone. of M not bound to edta] x (Cone. of edta not bound to M)

= [M]

1698

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

At the equivalence point, concentrations of Ca2+ and free edta are determined by the dissociation of the 1: 1 complex. It leads to: [Ca2+] = molar concentration of Ca2+ not bound to edta = molar concentration of edta not bound to Ca2+ = [edta] , l3[ca(edlal]

5xl0-3

= [ca 2 + f =1.75 x 10 [Ca 2+ ]

=(

3 5 X10- JI/2 "" 5.3 X 10-7 M 1.75 X 10 10

ra ry

or,

10

em

yl ib

i.e. pea = 6.27. (v) At VL = 100.1 mL (i.e. just after the equivalence point): [Ca2+] is determined by the dissociation of the complex. [edta] = conc. of edta not bound to Ca2+ = 0.1 x 0.01 M = 4.99 x 10-6 M

ch

100 + 100.1

3

13' [Ca(edta)]

4.99 x 10[Ca 2 +] x 4.99 X 10-6

M=~M=l 75xlO IO M [Ca 2 +]

e/

=

th e

al

[Ca (edta)2- ] = 100 x 0.01 M = 4.99 X 10-3 M 100 + 100.1

.

er

e

t.m

i.e. [Ca2+] = 5.7 x 10-8 M i.e. pCa = 7.24 (vi) At VL = 110 mL: [Ca2+] is determined by the dissociation of the complex.

= 10xO.Ol

100 + 110

H

[edta]

M=4.76xI0-4 M

C

lic

k

[Ca(edta)2- ] = 100 x 0.01 M = 4.76 x 1O-3 M 100 + 110

13'

= I 75 X 10 10 =

[Ca(edta)]'

3

4.76 X 10M = _10_ M [Ca 2 +] x 4.76 X 10-4 [Ca 2 +]

i.e. [Ca 2+]=

10 =5.7xI0-IOM 1.75 x 10 10 i.e. pCa = 9.24 Illustration 2: During the titration of 100 mL of 0.01 M M2+ by 0.01 M edta solution, the reaction is supposed to be almost complete for the addition of 99.95 mL of edta solution. Then pM changes by 3 units for the addition of excess 0.1 mL (i.e. 2 drops) of edta solution. Calculate the conditional stability constant, f)' M(edta)· Solution: VL = 99.95 mL

M2+ = []

om (100 - 99.95) M = 2.5 X 10-6 M 100 + 99.95

1699

ApPLICATION OF COMPLEX COMPOUNDS

i.e.

pM = 5.60 ~pM

= 3.0 i.e. pM = 5.60 + 3 = 8.60 when VL = 100.05 mL

It leads to: [M2+] = 2.50 x 10-9 M.

[M(edta)2-]", lOOxO.01 M=5xlO-3 M 100+100.05 [edta] = conc. of edta not bound to metal when VL = 100.05 mL

~'

=

[M(edta?- ] [M 2+ ]

x conc. of edta not bound to M 2+

em

yl ib

[M(edta)]

ra ry

0.05 x 0.01 M = 2.5 x 10-6 M 100+ 100.05

2.50 X 10-9 x 2.5 X 10--{}

ch

=8xl0 11 •

th e

al

11.7.5 Effect of other Complexing Agents on Complexometric Titration Curves: Effect of other Complexing Ligands on the Conditional Stability Constant

er

e

t.m

e/

Sometimes, it may be required to raise the pH significantly high but at this higher pH, the metal ion to be titrated may form the precipitate of hydroxide or oxide. To avoid this, it may be required to use the masking agent. When NH4CI-NH3 buffer is used to maintain the pH close to 10, NH3 as a complexing ligand can act as a masking agent. Thus NH3 functions dual roles: buffering action to give the required pH and masking agent action. In fact, for titration of Zn2+ by edta in presence of NH4CI + NH 3 buffer, the following reaction occurs.

H

[Zn(NH 3)4]2+ + Hedta 3-

r=

[Zn(edta)]2- + 3NH3 + NH4+

C

lic

k

In such cases, the extent of progress of the reaction depends on two factors: pH and concentration of NH 3 • In other words, conditional or effective stability constant of the [M(edta)] complex depends on both [H+] and [NH 3]. Dependence of ~[M(edta)] on [H+] arises from the competition between 'M' (i.e. metal ion) and H+ for the complexometric ligand (L) edta (assuming no hydrolysis of the metal ion). It is expressed in terms of a v Dependence of ~'[M(edta)] on [other complexing ligand] i.e. [NH 3] (in the present case) arises for the competition between NH3 and edta for the metal centre. The effect of such auxiliary complexing ligand (L') is expressed in terms of aM.

Thus a L takes care of distribution of the complexometric ligand (L) while aM takes care of the distribution of the metal centre among the different ligands. We have already discussed the origin and developmerit of av Now let consider the origin and development of aM. We can write: TM (concentration of total metal)

= Concentration of the metal bound to the complexometric ligand (L i.e. edta) + concentration of the metal bound to the auxiliary ligand (L') + concentration of M not bound to Land L'.

1700

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

i.e.

TM

[M(edta)] = ([M] + [ML'] + [ML'2] +[ML'3] + ...)

-

x

=I

[ML;]

i=O

x

or, TM

-

[M(edta)] =

I[ ML;] = i=O

Concentration of the metal not bound to the complexometric ligand (L i.e. edta)

= Zn2+, and L' = NH3, then we can write:

If we take: M 4

I[M(NH 3 );] = [M] + [M(NH 3)] + [M(NH3h] + [M(NH 3)3] + [M(NH 3)4]

ra ry

i=O

yl ib

= [M] + K 1[M][NH3] + K 1K 2[M][NH3]2 + K 1K 2K 3[M][NH 3]3 + KIK2K3K4[M][NH3]4 = [M] {I + K 1[NH3] + K 1K2[NH3]2 + K1K 2K 3[NH3]3 + KIK2K3K4[NH3]4}

em

=[M]{ 1 + ~/3n [L'r } , (n = 1, 2, 3, 4 in the present case)

/3

Cl. M

- [M(edta)] _ [M(edta»)-

[M][edta 4 -] -

-f A

[

+ ~""n L

,]n

[M(edta)]Cl.MCl. L {TM -[M(edta)]} {TL.,-[M(edta)]}

-

Cone. of [M(edta)] complex (Cone. of metal not bound to edta) x (Cone. of edta not bound to metal)

k

[M(edta)] -

H

13'

er

e

=~[M(edta)]aMaL

=1

al

[M]

th e

[M (edta)] =

-

e/

TM

t.m

It leads to:

ch

= [M]aM

= 9.0 (Table

11.7.3.1). Zn 2+ + NH 3 ~ [Zn(NH3)]2+, K 1 = 190 [Zn(NH3)]2+ + NH 3 ~ [Zn(NH3)2]2+, K 2 = 210 [Zn(NH 3)2]2+ + NH3 ~ [Zn(NH3)3]2+, K 3 = 250

C

at pH

lic

Illustration: ~[Zn(edta)] = 3 X 10 16 ; pH =9.0 maintained by 0.1 M NH 3 + 0.17 M NH4CI, a L = a edta = 19.0

[Zn(NH 3)3]2+ + NH 3 ~ [Zn(NH 3)4]2+, K 4 = 110 It leads to:

aM

= 1 + K 1[NH3] + K 1K2[NH3]2 + K 1K2K 3[NH3]3 + KIK2K3K4[NH3]4

= 1 + 190 ~ 0.10 + 190 x 210 x (0.10)2 + 190 x 210 x 250 x (0.10)3 + 190 x 210 x 250 x 110 x (0.10)4 ~ 1.20 X 105 , (using K 1 = 190, K 2 = 210, K 3 = 250, K 4 = 110) A'

_

fJ[M(edta)] -

~[M(edta)] a a M

L

3 x 10 16 - - - - - ~ 1.30 X 10 10 , (a L = 19.0, at pH = 9.0) 19.0 x 1.20 x 10 5

Thus, the conditional stability constant is widely different from that of the thermodynamic stability constant.

1701

ApPLICATION OF COMPLEX COMPOUNDS

It is evident that if [NH31 is increased, f3/[M(edta)] decreases. Consequently, change of pM at the equivalence point becomes less. It makes the titration less accurate. Effect of buffer on the complexometric titration: Buffer is required to maintain the required pH in a complexometric titration. But if the buffer contains the complexing ligands that can complex with the metal ion to be titrated, then conditional stability constant (i.e. ~/[M(complexometric ligand)]) decreases. Consequently, the extent of change of pM at the region of equivalence point becomes less i.e. change

of pM value at the equivalence point is less sharp. Illustration: 100 mL of 0.01 M Zn2+solution is titrated with 0.01 M edta solution at pH 9.0 maintained by NH 3 + NH4CI buffer. Calculate the change of pZn at the equivalence point for (i) [NH3] and (ii) [NH3] = 0.1 O. X

yl ib

Solution: We have: ~[Zn(edta)] = ~[Zn(edta)]aZnaedta

ch

em

(In general: ~[M(L)] = ~[M(L)]a~L)· aM (i.e. a Zn ) is given by:

= 1 + K l [NH3] + K 1K2[NH3]2 + K 1K2K 3[NH3]3 + KIK2K3K4[NH3]4

For [NH3]

= 0.01 M; aM

=1 +

aM

190 x (0.1) + 190 x 210 x (0.1)2 + 190 x 210 x 250 x (0.1)3 + 190 x 210 x 250 x 110 x (0.1)4 = 1.20 X 105 = 1 + 190 x (0.01) + 190 x 210 x (0.01)2 +190 x 210 x 250 x (0.01)3 + 190 x 210 x 250 x 110 x (0.01)4 . = 27.82

t.m

M;

16

3x10 zl.30x10 10 ( [J) 1.20x10 5 x19.0 ' atpH-9.0and NH 3 -O.lM

e

A

A'

al

= 0.1

th e

For [NH 3]

e/

aM

=

PZn(edta) aZnaedta

=

~[Z

3 X 10 = 27.82 z x 19.0

H

er

P[Zn(edta)]

C

lic

(d )] n e ta

k

16

I

Case-I (in presence of [NHal

13

5.6 x 10 ,(at pH =9.0 and [NH 3

J= 0.01 M)

=0.1 M)

(i) VL = 99.9 mL of 0.01 M edta solution (i.e. just before the equivalence point). Cone. of Zn2+ not bound to edta = Tzn - [Zn(edta)2-] = 0.01(100-99.9) M

100+99.9 = 5 x 10-6 M .. T -[Zn(edta)2- ] -_ --1 2 10 5 B Y de fiInltIon, Zn [ ] a Zn . x 2 Zn + -6

i.e. [Zn 2 +] = 5 x 10 1.2 X 10 5 and pZn = 10.38

M,

10 16 ; step-wise stability constants for Zn2+-NH 3

ra ry

Given: At pH = 9, a L = a edta = 19.0; ~[Zn(edta)] = 3 system, K l = 190, K2 = 210, K3 = 250, K4 = 110

= 0.01

= 4.16 X 10-11

1702

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

(ii) For VL = 100 mL (i.e. at the equivalence point).

[Zn(edta)2- J== 100xO.01 M =5x10-3 M 100 + 100

[Zn(edta)] -

[Zn(edta)2-

(cone. of Zn

2

+

notJ x (cone. of edta not)

bound to edta _

bound to the metal [Zn(edta)2-

- {Tzn -[Zn(edta)2- J}{T

edta

10

=

-[Zn(edta)2-

J}

[Zn(edta )2-]

=X (say)

5 X 10-3 2

em

,

J =Tedta -

At the equivalence point, TZn - [Zn( edta )2] Thus: ~[zn(edta)] =1.3x10

J

ra ry

_

yl ib

~'

X

ch

3

x 2 = 5 X 10- = 3.84 X 10-13 1.3 X 10 10 i.e. x = 6.2 X 10-7 M By definition:

e/

th e

al

or,

t.m

TZn -[Zn(edta)2- ] = = 1 2 10 5 cx. Zn • x 2+ [ Zn] = [Zn 2+] x 1.2 x 105

or,

6.2 X 10- = [Zn 2+ ] 1.2 x 105

er

e

or,

and (iii) For VL

= 5.16 x pZn = 11.28

[Zn2+]

C

i.e.

lic

k

H

7

10- 12 M

= 100.1 mL (i.e. just after the equivalence point).

[Zn(edta)2- J~ 100xO.01 M==5x10-3 M 100 + 100.1 Cone. of edta not bound to the metal =T edta - [M(edta)] 0.01(100.1-100) 100+100.1 = 5 x 10-6 [Zn (edta)2-

,

~[zn(edta)] ={ 2- J}{ T [ - Zn(edta) T Zn

J

edta -

[Zn(edta) J} 2-

1703

ApPLICATION OF COMPLEX COMPOUNDS

3

1.3 X 10 10 =

or,

5 X 10-

{Tzn -[Zn(edta?-J}x5 x 10-6 TZn

or,

( ) Znedta

- [

J=

2-

TZn -[Zn(edta)22+ [ Zn]

5 x 101.3 x 10

10

3

x 5 x 10

-6=7.7xl0

-8

J__ Vsp3-C-H i.e.,

V C - H : V==C-H)V=CH 2

)V -CH 3

With the increase of s-character of C-orbital in the C-H bond, the bond becomes stronger (i.e. shorter).

1736

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

VH- F =4100 em-1, VH- H =4160 em-1 : Effect of two light atoms, small bond length and high force constant. (ii)

v (2000-2500 em-I): Stretching frequency of the triply bonded systems (e.g. VC==N =2200 - 2300 cm -1; VC==C = 2225 cm -1 ) lies in this range.

(iii) V (1600 - 2000 em-I): Stretching frequency of the doubly bonded systems (e.g.

yl ib

ra ry

VC=O = 1600-1750cm- 1 , VC=C = 1650cm-1 , Vc N = 1600 cm- 1 ) generally lies in this range. Here it may be noted that frequency for the bending mode of vibration of the N-H group also lies in the given range of frequency (cf. for the stretching mode of vibration of the A-H bonds where A = C, N, 0, etc. the frequency lies above 2500 cm- I but for their bending modes, the frequency lies far below except for the N-H bond).

3000

2500

2000

1500

al

3500

cm-1

1000

t.m

e/

th e

4000

ch

em

(iv) Skeletal vibrations (Fingerprints): For the organic compounds, the skeletal vibrations give several absorption bands in the range 700-1400 cm- I . These absorption bands are called the fingerprints because from these bands, the skeletal molecular structure can be recognised. In fact, these bands are highly typical of the specific molecular skeletal structure.

Stretching

H

N--H

C

lic

k

C--H

Region-I:

C_C

er

O--H

e

Stretching

C

N

S--H, P--H, Si--H C--O

IV

I Stretching C===C

C===O C===N N===O P===O Bending N--H

vX-

Stretching C-C, C-N, C-O,C=F, C-CI,C=S Bending C-H,C-O, O-H Other stretching, bending and combination bands. The fingerprint region.

H (X = C, N, 0), single bond involving the light H-atom.

Region-II: vC==N' vC==C '

VX-H

(X = S, P),

Ve-o

Region-III: vx=y (X, y = C, N, 0, P) i.e. stretching of double bouds;

ON-H

Region-IV: Bending vibrations (N-H exception), weaker single bonds and fingerprint region.

Fig. 12.1.4.1 Regions of ir-absorption corresponding to the stretching (v) and bending (8) vibrations of the main functional groups and carbon skeleton bonds.

1737

SPECTROSCOPIC METHODS AND OTHER PHYSICAL METHODS IN CHEMISTRY

Table 12.1.4.1 Characteristic stretching frequencies of some molecular fragments Approximate frequency

Group

Approximate frequency

Group

(v in em-I)

(v in em-I) 3600 )C-O-

1100 - 1250 (vco )

3350 - 3400

)C=O

1600 - 1750

3300

\..C-C/ / \..

1650

3050

)C=N-

yl ib

@-H

(3200 - 3600) for H-bonded

ra ry

-OH

-N=C=O

1600

2270

em

(isocyanate)

-N=C=S

3030

2125

ch

(isothiocyanate)

1050

al

)S=O

th e

(sulfoxide)

-2930*

e

) CH 2 (methylene)

t.m

-2875**

e/

-2965*

-CH 3 (methyl)

~

C

lic

/ -SH

-2890 (weak)

k

-C - H (methine)

1000 - 1225

H

er

-2855**

\.. / -C-C/ \.. \.. / -C-N / \.. \.. -C-O/

2500 )C=S

\..pH / \.. -Si-H

2400

-C==N

2260

-C==C-

2225

\.. -P=O /

1140-1210

/

1075 - 1120

\..

2300

-C-F

/ \.. -C-CI

/ \.. -C-Br / \..

-C-I /

1050 750 660 560

(R3PO; Ar3PO)

*asymmetric stretch, ** symmetric stretch. Methyl (primary-C); Methylene (secondary-C); Methine (tertiary-C)

1738

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

~H.

1

\

--C-H

~

V (cm- ) (Symmetric stretch):

~H

~H

--N

~H

~H

H

-2855

~l-

\

~H

/

-C-H (Asymmetric stretch):

Amino

/C~

- 2875

V (cm-1 )

Methylene

C

~H

-N

~H

- 2965

~H

-2930

0. 0

0. 0

-N ·

~'o

-c·

~'o

-1350

- 3300

~H

Carboxylate Acid anhydride

Nitro

'i. a

-N ·

~·o

J}.o -C ·

~·o -1600

----+ c=o

/ 0"

C=O

/1~

-1400

-1550

- 3400

"

"

----+ C=o

cI

"/~o

C=O

ra ry

Methyl

yl ib

Note: The mode generating a larger dipole moment is of higher intensity

em

12.1.5 Electronic and Coupling Effect on Group Vibration Frequency These aspects are illustrated in different cases.

al

(),

th e

~C A/X

ch

A

vco :

(A) Inductive effect on

It indicates that the electron pushing inductive effect (i.e. +1 effect) introduces the single bonded

vco decreases.

e/

character into the CO group i.e.

t.m

vco : HCHO ) CH3CHO ) CH3COCH3

e

The electron withdrawing inductive effect (i.e. -I effect) enhances the veo value.

H

er

vco : CH3COCH3 90° or < 90°, the C-C single bond stretching vector can be resolved into two components and one of these two components is coincident with the direction of the C= C stretching vector and -it allows the coupling of C-C and C=C bond vibrations to raise the vc-c value.

i

I

~=900

~ P4

H

er

e

t.m

e/

th e

al

ch

em

yl ib

Electron donation from L makes L more electron deficient i.e. P4) P3' It enhances the ionic character· in the L-X bond, i.e. covalent-ionic resonance interaction to strengthen the L-X bond increases. It makes VL-X higher upon coordination. It happens so for the ligands like SnCIi (coordination through Sn), SOj- (coordination through S), CN- (coordination through C; n-acceptance of CN- is negligible), etc. VSn - CI (in free SnClj") =297 cm- I , 256 cm- I ; VSn - C1 (in [Rh(PPh3)3(SnCI3)] = 327 cm- I , 302 cm- I Some authors have explained the above observation in terms of the degree of covalence of the L...:.-X bond. This aspect is illustrated below. XL ( Xx: For XnL ~ M a-donation, L becomes more electronegative due to the increased electron deficiency and consequently the electronegativity difference, i.e. XL - Xx decreases upon coordination. It enhances the degree of covalence in the L-X bond upon coordination, i.e. vL-X increases upon coordination. XL ) Xx: For XnL ~ M a-donation, L becomes more electron deficient, and consequently, it becomes more electronegative. Thus the degree of covalence in the L-X bond decreases upon coordination because XL - Xx (electronegativity difference) increases. It reduces vL-X upon coordination.

lic

k

12.1.17 Application of Infrared Spectra: Shifting of Band Position to Characterise the Mode of Coordination - Illustrative Examples.

C

(A) Carbonato and nitrato complexes: In free carbonate (-1400, -1100, -880, -750 cm- I ) and nitrate, there is a partial double bonded character in the C-O and N-O bonds. But coordination through the O-end reduces this bond order. It is reflected in vc-o and vN- O which will be shifted towards the lower frequency upon coordination.

/0", M

C-::-=-::O ,(~'-

0' veo = 880 cm- 1

veo::::: 850 cm- 1

/0", M

C=O

"'0/ veo::::: 830 cm-1

1763

SPECTROSCOPIC METHODS AND OTHER PHYSICAL MFTHODS IN CHEMISTRY

The shifting of vc-o and vN-o can help us to understand their coordinating behaviours. It has been already mentioned that upon coordination, their local symmetry is reduced (D 3h ~ C2v , C 3v depending on the mode of coordination) and this symmetry lowering is reflected in the ir- and Raman spectra. These aspects have been illustrated in Sec. 12.1.19.

yl ib

ra ry

(B) Shifting of vc=o in the carbonyl compounds (see Secs. 9.4.11, 10.6.1): The nonbonding electron pair (which is weakly antibonding in character) is used for a-donation to the metal centre and the electron is received into the vacant 1t-ABMO through the n-bonding from the metal centre. Thus adonation, slightly enhances the C-O bond order (i.e. slight increase in vco) but the n-acceptance (which is more important) significantly reduces the C-O bond order. In most of the cases, effect of 1Ti-aceptance to reduce the C-O bond order is more significant than the effect of c;donation causing a slight increase in the C-O bond order. Compared to the terminal CO group, for the bridging CO groups, the reduction in the C-O bond order is more. M

2143

""'C===O

0 (terminal)

1850-2140

M

/

em

M-C _

CO (free)

1l2- CO

ch

1700-1850

t.m

e/

th e

al

Angular bridging CO groups (i.e. J.I.2-CO) vs. almost linear bridging CO group: When the metalmetal bond distance is relatively short, the linear terminal CO groups can act as the poor bridging ligands through a slight bending. The vco values of such nearly linear bridging CO groups are higher than those of the angular bridging CO groups (i.e. J.l2-CO). But the veo values of such nearly linear bridging CO groups are less than those of the typical terminal CO groups. Thus in terms of the vco values, the nearly linear bridging CO groups represent an intermediate situation and such CO groups are described as the semibridging J.l2-CO groups, i.e.

H

er

e

veo : terminal CO (1850-2140 em-I) > nearly linear bridging CO > angular bridging CO (17001850 em-I). It is illustrated in the following example.

k

[(11 5 -cp) Mo (CO

)31 ~[ (11 -Cp) Mo (CO)2 ] + 2CO 5

C

lic

The vco values and structural features of the above two compounds are illustrated in Fig. 12.1.17.1.

co

OC

5I

o

CO

\ /5

(11 -Cp)Mo - - - MO(l1 -Cp)

c~~.

/ . ·o':,pC ··.~~1

(11 5 -Cp)Mo, / / \

OC

I

CO

",

CO

-

-1

veo = 1915,1960 em (terminal CO group) (a)

Nearly linear ~ bridging CO ......---... group

'< MO(l1 5 -Cp)

~

veo = 1859,1890 em-

,

OC~d ~

Me

Me

0 ,. (Mo)d ~ n;*(CO)

1

(pure terminal CO group?) (b)

Fig. 12.1.17.1 (a) Structure of [(1l5-Cp)Mo(CO)3h; (b) structure and bridging property of the CO group in [(1l5-CP)MO(CO)2h.

1764

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

The relatively low veo values in [(1l5-Cp)Mo(CO)2]2 cannot be explained by considering the existence of pure terminal CO groups as in the proposed following structure satisfying the 18e rule.

co co I

I

I

I

(1l5-Cp) Mo_Mo (115-Cp) CO

(satisfying the 18e rule; no bridging property of the terminal CO group)

CO

er

e

t.m

e/

th e

al

ch

em

yl ib

ra ry

The bridging property of the linear terminal CO groups is attained through a slight bending and the following interactions are attained. (i) the n-bonding electron cloud of the terminal CO-group can be donated to some extent to the other metal centre to reduce the c-o bond order. (ii) the n*-MO of the CO group can also receive the electron cloud to some extent from the filled metal d-orbital to reduce the c-o bond order further. Both the said actions to reduce the C-O bond order are relatively weaker than those in the angular bridging CO group. This is why, the c-o bond order in the nearly linear bridging CO group is greater than that in the J..l2-CO group but less than that in a pure terminal CO group (i.e. M-CO). . On heating, [(1l5-Cp)Mo(CO)3]2 loses some CO molecules with the concomitant increase in the metal-metal bond order from 1 to 3 (in conformity with the 18e rule). This shrinkage in the metalmetal bond distance allows the linear terminal CO group to act as a poor bridging ligand in [(1l5-Cp)Mo(CO)2]2· Here it is worth mentioning that [(1l5-Cp)Mo(CO)3]2 shows the fluxionality (see Fig. 10.9.6.5 in Vol. 2 and Sec. 10.11.3) through the participation of the CO group as the bridging J..l2-CO group. But in the equilibrium mixture, concentration of the structure having the angular J..l2-CO groups is negligible and the ir-data also support the fact that it predominantly exists in the structure having only the terminal CO groups. • For the nonclassical carbonyl complexes (i.e. carbonyl complexes of the highly electronegative and heavier congeners, e.g. Au, Ag, Pd, Pt, etc.) where M ~ CO n-bonding is not important, the veo increases slightly in the complexes due to M ~ CO (a-donation of the nonbonding electron pair having

H

some antibonding character).

C

lic

k

Different factors to determine the v eo values in the carbonyl complexes have been dissussed with a large number of exampl~s in Chapter 9, Sec. 10.6.1 and Sec. 12.8.5, Vol. 3. (C) Nitrosyl complexes (see Sec. 9.6): Depending on the conditions, NO can form the bent (M-N

i.e. NO as ·NO-) and linear (M-N=O, i.e. NO as NO+) bonds. They can be distinguished

~

o

by comparing the

VNO

frequencies.

/\.

M-N-O

(ca. 170-180°):

VN - O ~ 1660 - 1900 cm- l

(ca. 120-140°) :

VN - O

(Linear) /\.

M-N-O

~

1525 - 1680 cm- l

(Bent) V NO

(J..l3-NO) Mn I

Complex: -V

CN

( em -1) :

[yII(CN)6r- [yIII(CN)6r2065 Increasing trend ofL

~

2077

M (J-donation; decreasing trend ofM~1t* (CN) bonding; electronegativity order: V III >V II >V I .

1768

Complex: -V

CN

( em -1) :

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

[Ag(CN)4]32092

[Ag(CN)3]2-

[Ag(CN)2]-

2105

2135

Increasing trend of ligand ~ metal cr -donation per CN- ligand; increasing trend of M -nt *-MO (CN- ) bonding per CN- ligand; decreasing number of CN- ligands; order of eN values is determined by the M ~ CN- sigma donation

v

ch

em

yl ib

ra ry

Note: • M ~ n*-MO(CN-) lowers the VCN value, i.e. better the n-acceptance, lower the VCN value. Higher electronegativity of the metal centre disfavours the M ~ n*-MO (CN-) n-bonding. Thus the trend of vCN is also in conformity with the trend of n-acceptance by CN-. But the resultant effect is mainly controlled by the M ~ CN- C1-donation not by M~N- n-back bonding. • In CN-, the C-end is less electronegative than the N-end. The electron pair used in a-donation is weakly antibonding in nature. Consequently, the a-donation from the C-end will raise the VCN value. This aspect has been already discussed in Sec. 12.1.16. • CN- bridged polymeric complexes: In such cases, there are two types of CN- groups - terminal CN--group (i.e. M-C-N) and bridging CN--group (i.e. M-C:=N-M). In such cases two different C:=N stretching frequencies are noted. In the polymeric complex of Na2[CO(CN)s], the corresponding values are: VC==N (terminal) = 2205 em-I) VC==N (bridging) = 2130 cm- l

th e

al

(I) Isocyanido complexes (see Sec. 9.8): Depending on the condition, it can form the bent C-N-C segment or linear C-N-C segment. The position of ir-stretching frequency (v NC ) can distinguish the bent and linear linkage mode of coordination.

n.

e/

+

jr'

-

~

M===C===N

t.m

R-N_C: (Free RNC)

v:::::: 2140 cm-1

"'R

+

-M-C==N-R

linear C - N -C segment

er

e

angular C - N -C segment

(M-CNR)

(M-CNR)

k

H

\INC: Bent C-N-C linkage < Linear C-N-C linkage

C

lic

The behaviour of aryl isocynadides (ArNC) is different from that of alkyl isocyanides (RNC). All these aspects have been discussed and explained in Sec. 9.8. (J) Effect of conjugation arising from the metal-d orbital with the unsaturation in the ligands: In the acetylacetonate complex, the following type conjugation may arise.

(pseudo-aromaticity in the ring through the involvement of the metal d-orbital)

Better conjugation stabilises the complex more. Thus with the increase of stability of the complex, the M-O bond strength increases and the C-C and c-o bond strengths decrease. It is illustrated in the following acetylacetonato-complexes.

1769

SPECTROSCOPIC METHODS AND OTHER PHYSICAL METHODS IN CHEMISTRY

Cu(ll) 15.0

Ni(ll) 10.5

Co(ll)

9.5

Pd(II) 27.0

Increasing stability of the M- 0 bond

-V

CO

( cm -1) :

1602

1592

1554

1545

Decreasing trend of the C-O bond strength

-V

C- C

(-1) cm :

1601

1592

1580

)

1570

Decreasing trend of the C-C bond order

)

ra ry

In fact, the better conjugation with the metal d-orbital reduces both the C-O and C-C bond order.

Co(ll)

Fe(ll)

al

Increasing trend

Cu(ll)

Zn(ll)

VM- N (cm- I ) (for bpy):

423

264

286

297

280

VM- N (cm- I ) (for phen):

530

th e

Electronegativity of M (II) :

Ni(ll)

ch

M(II):

em

yl ib

• Bipyridine and phenanthroline complexes: Let us illustrate the effect of metal ~ ligand (e.g. bpy, phen) n-bonding on V M - N • Obviously, the better metal~bpy/phen n-back bonding will strengthen the M-N bond (i.e. stability of the complex will increase). The less electronegative metal will act as the better n-donor to strengthen the M-N bond. It is illustrated in the following complexes.

299

300

288

288

e

t.m

e/

The electronegativity of M(II) (measured by the ionisation energy of M(II) ~ M(III)) roughly increases along the period of the d-block metals, i. e. Fe(ll) is the least electronegative centre in the above series. It makes the most favourable situation for M(II) ~ bpy, phen back bonding and it makes the VFe(II)-N highest and the vZn(II)-N lowest.

H

er

(K) Shifting of group frequency upon coordination by some typical ligands (e.g. urea, thiourea, dimethyl sulfoxide, triphenylphosphine oxide, etc.) (cf. Sec. 12.1.18):

C

lic

k

(a) When urea coordinates through the O-end, veo decreases but when it coordinates through the Nend, veo increases (effect of the resonance effect and electronegativity effect, cf. Secs. 12.1.5, 12.1.16, 12.1.18). (b) When dimethylsulfoxide coordinates through the O-end, vso deceases but vso increases when it coordinates through the S-end (effect of the change in the n-bond of the SO group and also the electronegativity effect of the donor atom). (c) Coordination through the O-end of triphenylphophine oxide and pyridine-N-oxide, reduces the v po and VNO values respectively (effect of both the n-bonding and electronegativity of the donor atom).

f\()P~: __ M

~-/ \ Ph

Ph

(d) Coordination by the S-site of SOl- raises the vso value but coordination by the O-site reduces the vso value (effect of the electronegativity of the donor site).

1770

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

12.1.18 Application of Infrared Spectra: Identification of Linkage Isomers The ambidentate ligands like SCN-, N0 2-, CN-, S03 2-, S2032-, etc. can produce the linkage isomers which can be identified by using the ir-spectra of the complexes. (A) Linkage isomers of SCN- (Sec. 2.2.5): The possible linkage isomers (S-bonded and N-bonded) are: M-NCS (thiocyanto complex) and M-SCN (isothiocyanto complex). The thiocyanto complex experiences the higher crys~al field strength and consequently the d-d transition occurs at the shorter wavelength compared to the case of the isothiocyanato complex. They can also be distinguished from their ir-spectra. Their resonating structures are given below.

.r

~~r-:._ M~N===C~ s: . . .~--i~~

/

••

2-

sp ••

N===C===S: .....~--i~~ M===N===C=== S:

II

III

yl ib

M 1\

(M -

+r

Sp2

ra ry

sp

C::=: 1800 -170 0 i.e., almost linear linkage)

N-

0.

+

' S - C - N .... ~~--i~~

/..

-

M

/~

-M

1\

( M - S - C : =: 1000 i.e., bent linkage)

~s~

IV

e/

III

al

S===C===N:

th e

sp3--.........0

ch

em

The bond angle value indicates that the resonating structure II is less important. The stretching frequencies indicate that the resonating structure III contributes predominantly.

C'

er

_.0

e

t.m

It indicates that the resonating structure III is more important. In the structure III, the C-S bond predominantly bears the single bonded character while the C-N bond predominantly bears the triple bonded character.

H

:s·'C:=N:.

..' S===C===N:

(Free ligand)

VI

k

V

~

C

lic

The stretching frequencies, VC-S = 750 em -1, VC-N = 2050 em -1 indicate that the resonating structure V is more important. • It is evident that for the S-bonded complex, i.e. M-SCN, the C-S bond predominantly bears the single bonded character while for the N-bonded isomer, the C-S bond gets the double bonded character predominantly and the triple bonded character of the CN-group is decreased. • For the S-bonded isomer, the triple bonded character in the C-N linkage is more important than that in the N-bonded isomer. These are strongly reflected in the v c- s and V C- N stretching frequencies (cf in free CNS-: c- s = 750 em-I, VC - N = 2050 em-I, 8NSC i.e. bending mode = 480 em-I).

v

N

M-NCS

M-SCN

vc-s (em-I):

III

780 - 860

690 - 720

C

VC - N (em-I):

( 2000 (broad)

) 2000 (sharp)

0NCS

(bending):

450 - 500

400 - 440

~ridging mode: I V C - N = 2150 - 2180 cm-

I /8",M

M

1771

SPECTROSCOPIC METHODS AND OTHER PHYSICAL METHODS IN CHEMISTRY

The relative positions of vc-s and vC-N (with respect to those of free SCN-) for the different modes of coordination by SCN- can be understood by considering the resonating structures of SCN-. For the M-NCS linkage, VC-N slightly decreases but vc-s increases compared to those of free SCN-. For the M-SCN linkage, VC-N remains almost unchanged and vc-s slightly decreases upon

coordination. Among the above three modes of vibration, vc-s is very often used as the diagnostic one for characterising the linkage isomers. This is illustrated in the following examples. Complex

2107, 2114

715

For KSCN

ra ry

K2[Hg(SCN)4]

vc - s = 750 cm- 1

2045, 2080

821,816

yl ib

(soft-soft interaction)

VC - N

=2075 cm- 1

em

trans- [Pd(AsPh3)iSCNhl

-700

ch

855

2120

th e

al

2090

e/

In the same way, the linkage isomers produced by SeCN-, and OCN- can be distinguished. Examples of the linkage isomers for SCN- are given in Sec. 2.2.5.

C

lic

k

H

er

e

t.m

(B) Linkage isomers for NOi: This ambidentate ligand gives the two isomers: N-bonded (M-N0 2, called nitro-isomer conventionally) and O-bonded (M-ONO, called nitrito-isomer conventionally). The nitrito isomer experiences a lower crystal field strength and consequently the d-d transition occurs at the longer wavelength compared to that of the nitro isomer. They can also be distinguished from their ir-spectra. The N0 2- (free) ion, has the low symmetry (C 2v ) and its three vibrational modes, i.e. symmetric and asymmetric stretching, i.e. V S (N~O) and va (N-O) and bending (8) are all iractive. The number of ir-bands do not change on coordination, i.e. vs' va and 8 bands remain present in both the free ligand N0 2- and the complexes. Thus the number of ir-bands cannot distinguish correctly the linkage isomers but the positions of the ir-bands can distinguish the isomers.

~9 M-N

.... 124 pm

~9

M-N

Vs

"',

300-450 cm- 1 )

.... 129 pm (.... 1000 -1100 cm-1)

M" ,\ )

I

o

(VM-N::::

I

- ~o ~9

M-N -

I

~,

....121 pm

--l

O-N~ o

(-1400-1500cnr1 )

",0

Va

(Relatively larger angular volume, i.e. more steric factor)

(Relatively smaller angular volume, i.e. less steric factor)

1772

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

NOi(free)

M-N02

M-DNO

N-O (pm):

.... 124

.... 121 (terminal) .... 129 (internal)

VS (N0 2 )(em- 1)

1310 - 1340

....1250

Va (N0 2 ) (em-I):

1370 - 1450

-1335

Pw(N0 2) (em-I):

620 - 640

(absent)

cS(ONO) (em-I):

....830

-830

....830

1040 - 1100

Va (ONO) (em-I):

1420 - 1480

S

60 - 120

ra ry

V (ONO) (em-I):

85

380 - 480

The important and diagnostic observations are:

em

yl ib

• For the M-N0 2 linkage isomer, both v lN0 2) (i.e. symmetric stretching band) and v a (N0 2) (i.e. antisymmetric stretch) are shifted towards the higher frequency significantly compared to those offree N0 2-.

(M-ONO) ) ) dV (M-N0 2). It indicates the more inequality of the two NO bonds in the nitrito isomer. In fact, for the nitro-isomer, both the N-O bonds are very much comparable and it makes d v smaller. In fact, d v for the nitro isomer is comparable to that of free NOi·

al

dV (= Vs - va): dV

e/

th e

•

ch

• For the M-ONO linkage isomer Vs is shifted towards the lower frequency while va is shifted towards the higher frequency compared to those offree NOi.

t.m

• The wagging mode, i.e. Pw(N02 ) at about 600 cm- 1 is absent in the nitrito isomer.

e

• The bending mode, i.e. 8(ONO) at about 830 cm- 1 remains more or less at the same position for the both isomers.

k

H

er

The above predictions are illustrated in the following linkage isomers.

= 500 cm- 1

C

VCo -

lic

(Yellow, higher lODq value) N

}

(Red, lower lODq value) (strong bands) Vs = 1065 cm- 1 (strong diagnostic band)

= 1315 cm- 1 Va = 1430 cm- I ~V = 115 cm- 1 Pw(N02) = 600 cm- I

= 1465 cm- 1 ~V = 400 cm- I Pw = absent.

Vs

Va

The bending vibration, i.e. 8(ONO) for both the isomers appears at about 825 cm- I. The magnitQde of ~v is a characteristic feature to distinguish the isomers. These are illustrated in the following examples (values are in em-I). Complex [CO(N0 2 )6P[Cr(NH3)5(ONO)]2+

Va (N -0)

Vs (N -0)

!J.V

cS(ONO)

v(M-N)

-1390

1330

- 60

835

415

1460

1050

410

840

absent

1773

SPECTROSCOPIC METHODS AND OTHER PHYSICAL METHODS IN CHEMISTRY

N0 2- can show its bidentate chelating action.

l1~_monomet~lliC} /~~N·

bldentate action

M...........

' '/

"0/

•

va (NO 2 ) ~

1270 em-I; V (NO ) 2 1

s

~ 1200cm-l,~v~70cm-l

O(ONO) :== 850 cmIt indicates that both the N-O bonds are almost equal.

H

/o~

M

j,~_-;-::: 0

/

(H 3 NbCo

Co(NH 3b

\\-1) \ 7~o

em

\

3+

yl ib

M

ra ry

For the bidentate chelating action, compared to the M-N0 2 mode of action, both the symmetric and antisymmetric stretching frequencies are lowered and the bending mode is slightly shifted towards the higher frequency. NOi can also function as a bridging ligand.

0'

ch

(trans-form) (1l2-bimetallic bridging action)

N---O

th e

al

/ o

Va (NO) ~ 1515 cm-1

t.m

cS (ONO) ~ 830 cm- 1

e/

VS (NO) ~ 1200 cm-1

H2

V(N02):

-1515 cm- 1, -1200 cm- 1

~

C

lic

k

H

er

e

/ N----CO(NH) (H aN)4CO " / a4 N-·-O

II

o

v (N0

(111-bimetallic bridging action) 2 ):

-1515 cm-1 -1200 cm-1

(C) Linkage isomers of 801-: SOj- can coordinate either through the O-end or the S-end. Oxygen is more electronegative than sulfur. Thus coordination through the O-end (i.e. more electronegative end) will reduce the vso stretching frequency (i.e. S-O bond will be weaker after coordination) (cf. Sec. 12.1.16). • vso (complex) ) vso (free SOj-) ~ Coordination through the S-end • vso (complex) ( vso (free SOj-) ~ Coordination through the O-end. Now let us consider the local symmetry of the M-S0 3 and M-OS0 2 linkages.

1774

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

For free SO~- (C 3v), the important ir-bands are:

= 961

VI (AI)

v (AI) = 633 cm-I; v (E) = 1010 cm-I; v (E) = 496 cm- I

C,?-I;

2

4

3

Out of these 4 ir-bands, the v3-band is the most sensitive one towards coordination, i.e. the V3 -band is considered as the diagnostic band to identify the coordinating behaviour of SOJ-.

v3(SO) (E-band):

C3v

C3v

Cs

1010 cm- I (free S03 2-)

) 1010 cm- I (for M-S0 3)

E-band splits into two components; ( 1010 cm- I (for M-OS02).

ra ry

It is illustrated in the following examples:

V3 = 1055 - 1080 cm- I ) 1010 cm- I

K 6 [Pt(S03)4] (S-bonded, monodentate): [Co(NH3)s(S03)]CI (S-bonded, monodentate):

v v

3

= 1110 cm- I ) 1010 cm- I

em

3

yl ib

(E) band spits: 862 cm- I and 902 cm- I ( 1010 cm-I.

TI 2[Cu(S03)4] (O-bonded, monodentate):

ch

(D) Linkage isomers produced by urea, N, N-dimethylformamide and N, N-dimethylacetamide: H

th e

/

Me 2 N

H 2N

"'c=o

al

"'c=o

i.e.

(N, N-dimethylacetamide)

DMF)

e/

(N, N-dimethylformamide,

/

Me 2 N

t.m

The important resonating structures are:

lic

k

H

er

e

Yeo = 1683 cm- 1

C

V eo = 1662 cm- 1 (cf.veo = 1685 cm- 1 (DMF);

veo ~ 1710 cm-

1

in

free carbonyl group)

The smaller value of v eo in urea, DMF or N, N-dimethylacetamide compared to that of acetone (-1715 cm- I) indicates that the nitrogen lone pair is engaged in resonance with the CO group in urea, DMF or N, N-dimethylacetamide. They can coordinate either through the O-end or N-end as follows:

'"

c=O

'"c===o: 0.

~

"'..7 N /

~

M ~

.

'"C - O.. - M ;

,,+~ N /

(More favoured)

"'/

M~:N

/

1775

SPECTROSCOPIC METHODS AI\JD OTHER PHYSICAL METHODS IN CHEMISTRY

• Coordination through the O-end: veo and v MO differ widely and consequently their coupling (called' kinematic coupling) to raise the vco value can be ignored. Removal of the 1t-electron cloud from the CO group weakens the C-O bond. In the most favoured resonating structure, the C-N bond gets the more double bonded character. Thus coordination through the O-end, will

(em-I):

1471

1725

1505

1395

1505

th e

VCN

e/

1683

}

U ==> urea

t.m

(em-I):

[CrU6]CI3

[PtU2 CI2]

U(free)

Vco

al

ch

em

yl ib

ra ry

raise the VCN value but lower the vco value (compared to those offree ligand, i.e. urea, DMF or dimethylacetamide). The NCO bending frequency will go to increase on coordination through the O-end. The C-O stretching frequency and NCO bending frequency are treated as the. diagnostic parameters because the C-N stretching frequency needs a complicated interpretation due to the coupling-effect. • Coordination through the N-end: The nitrogen lone pair is engaged in metal coordination. Thus the nitrogen lone pair is not available for participation in conjugation with the CO group. It enhances the double bonded character in the CO group and reduces the double bonded character in the C-N linkage. It will raise the vco value and lower the V CN value (compared to those of the free ligand) It has been found that in complexation of urea with the relatively harder metal ions like Fe3+, Cr3+, 2 Zn + or Cu 2+, the veo decreases and v eN increases. It supports the coordination through the O-end. But when urea undergoes complexation with the relatively softer metal.centres like Pd2+ or Pt2+, the vcois found to increase with simultaneous decrease ofv CN . It supports the coordination through the N-end. These are illustrated with the experimental data.

C

lic

k

H

er

e

(E) Linkage isomers of thiourea (coordination by the S-end or N-end) can also be distinguished in the same way as in the case of urea.

~cs ~ 730 em-1 (free ligand)

_ / M-S-C

NH

2

i.e.,

"-NH 2 +

It lowers the ves value but the deformation vibrations of the NH 2 do not change significantly.

(F) Linkage isomers of dimethylsulfoxide (Me2S=O): DMSO can coordinate either through the S-end or O-end. Obviously, the O-end is preferred for the harder metal centres while the S-end is preferred for the softer metal centres (e.g. PtU , Pdu). Coordination through the O-end reduces the vso value because the double bonded character in the S-O bond is destroyed partially upon coordination. Besides this, the O-site is more electronegative

1776

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

than the S-site. Thus coordination through the O-site also reduces the vso value (cf. Sec. 12.1.16). The coupling of v so and v MO to raise the value of vs o may be ignored because v so and v MO differ widely. Coordination through the less electronegative S-site will raise the vso value. It may be mentioned that in this mode of coordination, the n-bond in the SO group is not practically disturbed in resonance.

0

R

)s=o

(Free R2S=0, vs- o :::: 1050 cm- 1 , R=Me)

' "S + -O-

R

R

/

R\

M-;=0

R (vs-o >1050 cm-

1

R)S-~-M (vs-o !. 2

However, the elements such as 14N(I = 1),

(I = %). 1(I = %) are abundant and they are used in NMR spectroscopy. 127

1780

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

(C) Frequency for NMR transition and relation with the Larmor frequency: Nuclear spin can interact with the external magnetic field. In absence of the magnetic field, different spin states characterised by m/ (nuclear spin angular quantum number) which can have values I; (I - 1), ... , (-I + 1), (-I) (i.e. +1 to -I at intervals of unity) are degenerate. But this degeneracy is lifted in presence of an external magnetic field.

-+~ '-2 +1. ...:". m1--2 .:>' ':.--

..::--.

+1.

ra ry

No field

2

-----+~

yl ib

2

ch

em

1 3 Fig. 12.2.1.1 The allowed energy levels in presence of a fixed magnetic field (Ho) for the nuclei I = - and -. 2 2

2

opposes the field and it gets destablised to the same extent (Fig. 12.2.1.1.).

E=-m Y;n;h )Ho i.e. E, (m J(

E

mJ

..

Assuming the applied field (H 0) = field experienced (Hex); generally,

Hex < H 0 and in the actual expression o H 0 is to be replaced by Hex.

er

2(

H

and

+i) =-i( Y;n;h )Ho =-i) =+i( Y;n;h )H

=

e

J

e/

= _!

t.m

state m J

th e

al

For a nucleus of I = !, the spin state mJ = +! gets oriented along the field and consequently the 2 2 state becomes of lower energy. On the other hand, the nuclear magnetic moment of the nuclear spin

)

Nuclear magnetic moment Nuclear angular momentum

C

lic

k

YN (gyromagnetlc ratio of the nucleus = - - - - - - - - - -

i.e., V=(YN)Ho=gJIlNHOHZ 2n

In general, !'1E

h

h =Y21t N Hoflm J

gNIlNHo Hz, (representingg J bygN) h

where flm J = ±1; assuming applied field (Ho)

=experienced field

(Hex)·

In presence of a magnetic field, the nuclear magnetic moment (of the nucleus of nonzero I) precesses about the direction of the field (Fig. 12.2.1.2). The angular frequency of this precessional motion is

1781

SPECTROSCOPIC METHODS AND OTHER PHYSICAL METHODS IN CHEMISTRY

referred to as the Larmor precessional (circular) frequency (OlL) which depends on the magnetic field strength (Ho), (OL = yNHo. Larmor precessional frequency OlL (rad S-I), YN (rad S-1 1 1) and Larmor frequency (v L in Hz) are related as:

h

27

= 5.585 x 5.04 x 10-34 JT-

1

=5.585, IlN =5.04 X 10-27 JT- 1

xl T (

6.62 x 10- J s

say

HIT)

Fig. 12.2.1.2 Precession of a nuclear magnetic moment under the influence of an applied magnetic field (Ho).

=

"0

al

v

Thus,

g/

em

=hv = g/IlNHO i.e. v = g/IlN H O;

ch

M

yl ib

ra ry

Thus, the NMR transition will occur under the condition, VL = v, i.e. Larmor frequency (vL ) becomes just to equal to the frequency (v) related to the energy separation between the energy levels for the NMR transition. The energy difference (~) is quite small. It is illustrated for 1H.

th e

= 4.26 X 107 Hz = 42.6 MHz (a radiofrequency)

t.m

e/

It leads to: v = 63.9 MHz for H o = 1.50 T, v = 749 MHz for H o = 17.6 T. Thus with the increase of magnetic field (Ho), dE increases and V lies in the radiofrequency region.

NMR

VB.

ESR Transition

H

er

e

For the NMR transition, v lies in the radiofrequency region while for the ESR transition, v lies in the much higher frequency region (in the region of microwave frequency). This difference occurs from the difference in JlN and JlB.

lic

41tmp c

k

eh eh JlN = - - - andJlB = - - ; me 41tm e c

= 9.1

X

. 10-28 g whIle m p

= 1.67 x 10-24 g.

C

It makes, J.!B/JlN :::: 103. For the ESR transition, LlE 5.04 X 10-27 J 11). dE

. = gJlBHo, z.e.

hv

= gJlBHoLlms' (Llm s = ±1), g = 2; JlB = 9.27 x x

= gJlBHo and

10-24 J 11 (cf. JlN =

gJlBHO v = --h

24

1

v=2X9.27Xl0- JT- XIT (forH =IT) 6.61 x 10 -34 J s ' 0

= 2.8 X

1010 Hz = 28 GHz, (a microwave frequency)

In the NMR experiments, generally a fixed operating radiofrequency (say 60 MHz) is applied and then the magnetic field (Ho) is applied and gradually increased. When Larmor frequency (VL) becomes equal to the operating frequency, then resonance, i.e. NMR transition occurs. The condition for the NMR transition is: VL = v (operating frequency) where VL oc H o.

1782

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

12.2.2 Intensity of the NMR Signals and Peak Broadening (A) Probability of the upward and downward NMR transitions: The ratio of population density at two energy levels between which the NMR transition occurs is given by:

(-~)

n~

n

=exp kBT ; ~ 1- gIJlNHO kBT

For T = 298 K, kBT

~ 4.1

kBT

e

kBT

er

nl

~ ) = exp(-~), v = 60 X 106 Hz, kB = 1.38 X 10-23 JK- 1 molecule- l 2

x 10-21 J/molecule, the ratio

H

!!:.L = exp (-

t.m

e/

th e

al

ch

em

yl ib

ra ry

~ = gj J.lN H o crJ

al

0= (Hs;r

0' r _

l-O's'

em

Hs Hr

ch

i.e.

yl ib

ra ry

(Note: Ignoring the solvent effect and other minor effects, the main contributing factors to 0' are the local effect (O'd and O'p) and remote or neighbouring group effect (O'n) which also includes the ring curre~t effect, i.e. a == ad + a p + an where 0d (> 0) and op( < 0) denote the local diamagnetic shielding and local paramagnetic deshielding effect respectively. In PMR spectroscopy, O'd> O'p == O. All these aspects will be discussed later). NMR spectra are recorded by using a constant oscillator frequency, i.e. probe frequency or operating frequency (say 60 MHz) and varying the magnetic field (Ho). For the higher shielding constant (a), Hex (experienced field) is lower than the applied field (Ho) and the resonance will occur for the higher applied field for which v L (Larmor frequency) = yNH ej2n becomes the operating frequency (v). We can compare the value of H (say H r ) required to produce the resonance in the reference compound with the value (say H s ) required to produce the same effect in the sample compound. Hex = Hi1 - Os) and Hex = H r (1 - Or)' i.e. Hs (1 - as) = Hr (1 - O'r) where as and or denote the shielding constants for the sample and reference respectively.

e

t.m

e/

th e

• Reference in recording the PMR spectrum ofH-nuclei: For this purpose, TMS (tetramethylsilane, SiMe4) is used as the reference compound. All the protons of TMS are equivalent and the shielding constant is very high. They give a single peak at a very high field strength (for 60 MHz, H o = 1.4092 T). The 8-value for the TMS protons is arbitrarily set equal to zero. The larger value offJ indicates a downfield (i.e. lower field) resonance, i.e. lower shielding constant (a). In the tau (t = 10 - 8) scale, the reference is assigned the arbitrary position of 10.

H

er

• TMS as a reference one-why?: It is selected as a reference one because of its following characteristic properties. (i) The 12 equivalent protons give a single PMR signal.

C

lic

k

(ii) The electronegativity of silicon is very small (cf XSi == 1.8 ( Xc = 2.5) and consequently the protons are highly shielded (i.e. a is very high) and they resonate at a very high field (Ho = 1.4092 T for a 60 MHz probe). In fact, with respect to the TMS protons, almo~t all other protons (in organic compounds) resonate in the downfield direction. (iii) TMS is very much chemically inert and stable. (iv) It has a very low boiling point (300 K) and it can be easily removed by evaporation from the sample. (v) TMS is not water soluble. It is a drawback. For the water soluble sample, DSS (sodium salt of 2,2dimethyl-2-silapentane sulfonic acid), i.e. Me3Si(CH2)3S0i Na+ or (CH3)3Si(CD2)2COi Na+ is used as a reference one. CH3-protons of DSS resonate in the same position as those of TMS. • Other reference compounds: TMS generally used for IH, l3C and 29Si NMR studies; CFCl 3 for 19F NMR studies; 85% H3P04 or OP(OEt)3 for 31p-NMR studies. • a and fJ values for the different groups of protons in CH3CH20H: The proton of the OH group is least shielded as it is attached to the highly electronegative O-atom (i.e. a is small). Thus the magnetic field (Hex) experienced by the OH proton is more close to the applied field (Ho). This is why, among the

1789

SPECTROSCOPIC METHODS AND OTHER PHYSICAL METHODS IN CHEMISTRY

three types ofprotons, the OH-proton resonates at the lowestfield. On the other hand, the CH3-protons are most shielded (i.e. a is maximum) and they resonate at the highest field. The CH 2 protons resonate at an intermediate field. a CH3 ) a CHz )

i.e. Hex

00H

= H o (1- OCH 3 ) ( Hex = (1- 0CH z ) (Hex = H o (1- 00H)

(Hex )CH 3 < (Hex )CH z < (Hex )OH

i.e.

e/

th e

al

ch

em

yl ib

ra ry

Thus for the PMR signal H o : CH3 ) CH2 ) OH (for a particular fixed probe frequency) Similarly for CH 30H, H o : CH 3 ) OH For CH 3CHO, H o : CH 3 ) CHO • Relation among CJ (shielding constant), a (chemical shift) and Ho (required field for the NMR transition at a fixed probe frequency): From the cases of CH 3CH20H, CH30H and CH 3CHO, we can conclude as follows: (i) More the shielded protons (i.e. more the positive value of a):::::) more the electron density around the proton, more the upfield signal:::::) less the B-value or more the t value (for the TMS reference). (ii) More the deshielded protons as in the OH group proton (i.e. less the a-value):::::) less the electron density around the proton:::::) more the downfield signal:::::) more the B-value or less the t-value (for the TMS reference). (iii) For TMS, the PMR signals appear at 8 = 0, i.e. t = 10. (iv) a-scale (PMR): RCOOH*(10-13) R-OH* (2-5) R-NH 2* (1-3) R-CH 3* (0.6-1.2)

I

t.m

Negative shielding i. e. deshielding

Decreasing

8:

~

Increasing

H

't:

Increasing

lic

J

(Hex (Hal

(Hex '" Hal

8:

(for a fixed probe frequency)

Positive shielding

--+ Ho (applied field)

Downfield

~

~

k

Ho(app):

(Hex} Hal

~

er

0:

I

e

Increasing

No shielding or no deshielding

Upfield ~

Decreasing

C

't:

~

Increasing

TMS Deshielding

Shielding

Downfield

Upfield

4

8 (ppm): 10

9

8

7

6

5

4

3

2

Increasing

• Solvents used in PMR studies: Aprotic solvents (having no H-atom) producing no PMR signal are used for the solid and viscous liquid samples for their PMR studies. The common solvents ar~: D20,

1790

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

CCI4 , CDCl3(deuteriochlorofonn), CS 2, C 6D 6 (hexadeuteriobenzene), (CD3)2C=O (hexadeuterioacetone), (CD3)2S=O (hexadeuteriodimethyl sulfoxide), etc. It may be noted that the deuterium nucleus is NMR active (I = 1) but because of its very small value of YN (= 41.1, cf YN = 267.5 for 1H), its NMR frequency is small (cf 9.21 MHz for 2H and 60 MHz for IH in a magnetic field of 1.4 T). Thus the

NMR transition for deuterium in a 60 MHz spectrometer will appear at a very large magnetic field (about 9.0 T). This is why, no peak for deuterium will appear in the magnetic field used for scanning the NMR signal of tH.

12.2.5 Factors Controlling the Magnitude and Direction of Chemical Shift

ra ry

The important factors may originate from the both local and remote effects and each effect may lead to the diamagnetic shielding and paramagnetic deshielding, i.e. a ::::: alocal + aremotc (i.e. an) = ad +

ap + a w

e/

th e

al

ch

em

yl ib

(A) Local Diamagnetic Shielding (ad' an isotropic effect): Under the influence of an externally applied magnetic field (Ho), the electrons around the nucleus are induced to circulate to produce an induced magnetic field (Hinduced DC H o, i.e. Hinduced = adH o). This induced magnetic field opposes the applied magnetic field and the nucleus (to experience the NMR transition) experiences a less magnetic field. Obviously, the diamagnetic shielding constant ad is positive in the expression, Hex = H o(1 - a) where a ::::: alocal + aremote and al ocal = ad + a p (cf in PMP spectroscopy, cr local::::: cr d i.e. cr p ::::: 0). This phenomenon leading to crd is called the local diamagnetic shielding (which is identical in all directions, i.e. isotropic) whose magnitude depends on the electron density around the nucleus under consideration and the applied field strength (Ho). Electron circulation

H

er

e

t.m

Nucleus

Revolving electron

C

lic

k

Lines of the magnetic flux

Fig. 12.2.5.1 Local diamagnetic shielding leading to ad at the nucleus due to the electron circulation around the nucleus.

More the electron density around the nucleus, more the diamagnetic shielding, i.e. more the upfield NMR transition and less the chemical sh~ft (8). The magnitude of local diamagnetic shielding ofprotons is given by ad :=:: 21.51 x 10--6 where A gives the measure of effective number of electrons in the 1s atomic orbital of hydrogen. For the common type of protons, ad varies in the range 10~5 while it is in the order of 10-3 to 10-4 for the heavier nuclei. The following factors determine the magnitude of the local diamagnetic effects. (i) Electronegativity effect and inductive effect: The electronegative substituents show the electron withdrawing inductive effect to reduce the electron density around the nucleus. It reduces the local diamagnetic shielding (i.e. downfield NMR transition) and the chemical shift (8) becomes higher. It is illustrated in the following examples (for the PMR signals).

1791

SPECTROSCOPIC METHODS AND OTHER PHYSICAL METHODS IN CHEMISTRY

CH 3-F CH 3-OH CH 3-CI CH 3-Br

CH 3-X:

Si(CH3)4

= 2.5 = 2.2 X(Si) = 1.8

X(X):

4.0

3.5

3.1

2.8

X(C)

8 (ppm):

4.3

3.4

3.0

2.7

X(H)

8=0

Effect of substitution: CHCl 3 (8 = 7.27), CH 2Cl 2 (8 = 5.3), CH 3Cl (8 = 3.0) The electron pushing groups like alkyl groups enhance the electron density around the nucleus to increase the diamagnetic shielding (i.e. upfield transition and lower 8-value).

= 0.7 -

1.2) (R2CH 2 (8 = 1.2 - 1.4) ( R3CH (8 = 1.4 - 1.7).

ra ry

R-CH 3 (8

yl ib

(ii) Hybridisation state of C: The electronegativity sequence is: sp-C ) Sp2_C ) Sp3_C. Thus consideration of the local diamagnetic shielding predicts the chemical shift order for PMR.

'"

8 (ppm): -C - H (=CH 2

(==C - H (expected order based on

(Jd)

em

/

: 8 (ppm) = 7.32 (X = H), ) 7.32 (X = electron withdrawing group), ( 7.32 (X = electron pushing group)

: Order of deshielding of protons (0 ) p ) m);

C

lic

k

H

er

e

x-@-x

t.m

e/

th e

al

ch

But the above prediction is not supported experimentally because, another effect called magnetic anisotropy is important. This aspect will be discussed separately. Because of this anisotropic effect, the aromatic hydrogens, vinyl hydrogens and carbonyl hydrogens experience the downfield resonance (i.e. higher 8-values). (iii) Effect of resonance: Aromatic hydrogens generally show the 8 values in the range 7-8 ppm. In the benzene ring, electron pushing effect enhances the electron density to make 8 relatively less while the electron withdrawing groups make the chemical shift relatively larger.

n0 n

,-~::

8-"==./ 8-

: Order of sheilding of protons (0 ) p ) m);

NH 2

(iv) Hydrogen bonding: The hydrogen bonding, R-O-H···O (0 = donor, an electronegative centre) reduces the electron density around the H-centre, i. e. H-bonding reduces the local diamagnetic shielding, i.e. H- is deshielded. The H-bonded proton experiences the more downfield resonance, i.e. higher chemical shift, compared to the non-H-bonded protons. The intermolecular H-bonding of R-OH (in an aprotic solvent like CCI4 ) can be minimised by increasing the dilution. In fact, in dilute solution, the OH proton of ROH shows the chemical shift in the range 0.5 - 1.0 ppm but in concentrated solution (where the intermolecular H-bonding occurs), the chemical shift occurs at about 4-5 ppm.

1792

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

,

H

o

0

I

I

R

(Free, in dilute solution, 8 = 0.5-1.0 ppm)

H "

"/

'. /

H

0

/

" '

I

R

R

(Intermolecular H-bonding) (H-bonded ROH in concentrated solution, 8 = 4-5 ppm)

yl ib

ra ry

Obviously, in the case of intramolecular H-bonding, there is no effect ofdilution on the position of the PMR signal of the OB proton.

Enol form (~-diketone) 80H~ 10-13 ppm

em

(Salicylates, 80H ~ 10-12 ppm)

Intramolecular H-bonding

ch

Intramolecular

/H",

t.m

0

I C C

CH 3

H2

(Keto form)

lic

k

Two types of protons i.e. CH 3 and CH 2 (methylene) protons and two PMR signals of the intensity ratio 6:2

C

Existence of the

H3C

/'

(c)

. . . H,

o

0

C

C

I

~

"(a)

er

(b)

/

H

"

e

I C

H3 C

e/

th e

Acetylacetone:

(a) /

~-ketoester.

al

Keto-enol tautomerism: It occurs very often for ~-diketone and keto-enol tautomerism can be identified from the PMR signals.

o

R

O~

I

~C/ ' " CH I

H)

Methine proton (This methine proton can also be described as a

vinyl proton.)

I:,

i.e.

C (a) / 3

H3C

~O

.........

~ CI

"C/ I

(b)

"(a)

CH 3

H (enol-form) Three types of protons and three PMR signals of the intensity ratio, 6:1:1;

existence of the H-bonded proton and methine proton indicates the enol form.

Thus the PMR signals of a keto-enol tautomeric mixture show the different types of protons of different chemical shifts. For acetylacetone, the chemical shift values of the PMR signals are: 8 (ppm) (for the enol form) = 15.5 (OH), 5.5 (vinyl or methine C-H), 2.0 (CH 3); 8 (ppm) (for the keto form) = 2.2 (CH 3), 3.6 (CH2) For acetylacetone, the strong PMR signals appear for 8 = 15.5 ppm, 5.5 ppm and 2.0 pm while the weak PMR signals appear for 8 =2.2 ppm and 3.6 ppm. It clearly indicates that for acetylacetone, the enol form predominantly exists (about 80%). Thus positions of the PMR signal for the OH, NH, SH groups depend on the degree and strength of H-bonding which depend on the nature of solvent, concentration, temperature and characteristics of the substance. Greater the hydrogen bonding, greater the depletion of electron density

1793

SPECTROSCOPIC METHODS AND OTHER PHYSICAL METHODS IN CHEMISTRY

around the proton, greater the deshielding and greater the downfield resonance (i.e. higher chemical shift). (v) Effect of charge: Electron deficiency arises in the cationic species (i.e. deshielding effect and downfield resonance) while in the anionic species, the enhanced electron density causes the more shielding (i.e. upfield resonance). This is illustrated in the following examples.

Cycloheptatrienyl cation, 8 (H) ~ 9.2 ppm

[ CH 2 =-=-=CH=-=-=CH 2 ] 9.65

+

8.95

[

CH 2 =-=-=CH=-=-=CH 2 ] -

8 (ppm) = 2.45

6.30

2.45

em

8 (ppm) = 8.95

ra ry

©

Benzene 8 (H) ~ 7.2 ppm

yl ib

Cp- (cyclopentadienyl anion), 8 (H) ~ 5.4 ppm

All are aromatic, 4n + 2 = 6 i. e., 6 n-electrons

'"'-

----....

V

_---------J

ch

Allyl cation: Positive charge is more concentrated on the central carbon. Ally anion: Negative charges are more concentrated on

th e

al

the terminal C-centres to avoid the electrostatic repulsion. MOT can explain this charge distribution pattern.

t.m

e/

(vi) Acidic hydrogens of -C02" group, -803" group, etc.: Such hydrogens are the least shielded ones (i.e. approximately H+ ions). They show the downfield resonance (8 : : : 10 - 14 ppm). Both the electron withdrawing inductive and mesomeric effect make the hydrogen so electron deficient.

e

CO

C~-H

R-S=O

C~-H

lic

k

H

er

R-C-f"

yO

C

(B) Local Paramagnetic Interaction «(Jp, anisotropic in nature): This effect is mainly important for the nuclei like C, N, 0, F, etc. but this effect is not discussed here in detail. The applied field can cause the electrons to circulate using the low-lying unoccupied orbitals. Obviously, if the unoccupied orbitals are ofmuch higher energy then this effect is ofno importance as in the case ofhydrogen. Paramagnetic field at the nucleus causes deshielding (i.e. downfield resonance). Here the anisotropic effect arises from the mixing of the ground state and excited state leading to a field induced nonspherical circulation of electron around the nucleus. Here the word 'paramagnetic' is not used to imply the same property of paramagnetism arising from the unpaired electrons in a system. In the present case, it describes the field induced nonspherical circulation of electron density through the mixing of the ground and excited states. It makes the paramagnetic contribution to shielding (cf. the local diamagnetic shielding constant (Jd is positive while local paramagnetic shielding constant (Jp is negative). The contribution of (Jd and (Jp to 8 (chemical shift) can be understood in terms of Ramsey equation and Lamb equation (W.E. Lamb, 1955 Nobel Prize in Physics for the discovery of "Lamb shift" in the hydrogen spectrum) which are beyond the scope of the book. Qualitatively, it is concluded that the asymmetric distribution of electrons in the p- and

1794

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

d-orbitals at the both ground and low-lying excited states will be ideal for the paramagnetic contribution. The energy difference (~E) between the ground and excited states should not be large to prevent their mixing. CJd vs. CJp : (Jd opposes the applied magnetic field at the nucleus (i.e. the nucleus is shielded) while enhances the magnetic field intensity at the nucleus (i.e. the nucleus is deshielded). Diamagnetic shielding is favoured when the electrons are localised on a single atom or around a linear double or triple bond while paramagnetic deshielding is favoured when the applied field causes the electrons to circulate nonspherically throughout the molecule using the unoccupied orbitals. • CJd and CJp for IH vs. elements of higher atomic number (e.g. lIB, 13C, 19F, etc.): For hydrogen, to have the paramagnetic contribution, the electron is to be promoted to the 2p-orbital (cf s-orbital is spherically symmetrical) and this transition is energetically very much unfavourable because of the large ~E value. This is why, for IH, contribution of a p is insignificant. However, for the elements of higher atomic number, AE gradually decreases and contribution of CJp increases dramatically. For the heavier halogens, contribution of (Jp is more because of the smaller values of excitation energy (~). For IH, both (Jd and (Jp (::::: 0) are small because it involves fewer electrons and larger AE value and the chemical shift (8) covers a narrower range (ca. 20 ppm). For the elements of higher atomic number involving the larger number of electrons and smaller AE values, both (Jd and (Jp are quite high and (Jp dramatically increases with the increase of atomic number and it becomes the dominating factor to control the range of 8-values. For boron (i.e. lOB, lIB), both (Jd and (Jp are of comparable values and the 8-values cover a range of about 140 ppm. For l3C and 19F, (Jp is very high and the 8-values cover a wide range (cf about 225 ppm for l3C, about 1000 ppm for 19F). Thus we can conclude: CJp » ad (for 59CO, 14N, 13C, 19F); CJp ::::: CJd (for 10, lIB); CJd » CJp ::::: 0 (for IH); CJd (10, lIB, 13C, 19F) •

e/

th e

al

ch

em

yl ib

ra ry

(Jp

»

t.m

CJd (IH). • Chemical shift (8) range: ca. 1000 ppm (for 19F); ca. 225 ppm (for 13C, in organic compounds); ca. 140 ppm (for 10,11B); ca. 20 ppm (for IH).

C

lic

k

H

er

e

(C) Neighbour Anisotropic Effect (CJremote or an, Remote Effect) of the x-Electron Cloud: The n-electron cloud is also induced to produce a magnetic field in presence of an external magnetic field

~LineSOf

magnetic flux

Deshielding region

Deshielding region

l

-....y(a)

)

1795

SPECTROSCOPIC METHODS AND OTHER PHYSICAL METHODS IN CHEMISTRY

r

o orientation)

l____________

(Diamagnetic shielding effect on hydrogen)

(Paramagnetic (Perpendicular deshielding orientation) effect on carbon) ~

yl ib

H (Parallel

em

(Diamagnetic shielding H~ effect on both hydrogen Shiel,ding and carbon) region

ra ry

Shielding region

~

ch

(b)

Shielding region

th e

al

Deshielding region ~

e/

+

Deshielding ~ region

Circulation of 1t-electron

t.m

~

C

lic

k

H

er

e

Lines of magnetic flux

(c)

Fig. 12.2.5.2 Contribution of neighbour anisotropic effect (i.e. remote effect) from the induced circulation of tt-electron cloud in (a) ethylene, (b) acetylene, and (c) benzene.

due to the directed circulation of the n-electron cloud. This induced magnetic field may be diamagnetic or paramagnetic depending on the direction, i.e. anisotropic behaviour. The neighbouring nuclei which are not directly chemically bonded to the n-system lllay also experience the induced anisotropic magnetic field due to the circulation of the 1t-electron cloud under the influence of the external field. This is why, this effect is also described as the remote effect. A particular nucleus may be shielded or deshielded by this induced magnetic field depending on its orientation with respect to the n-system. This neighbour anisotropic effect (i.e. an) is also to be considered to estimate the magnetic field experienced (Hex) by the nucleus under investigation. The anisotropic magnetic field generated due to the induced circulation of the 1t-electron clvud is illustrated for alkenes, alkynes, aromatic system and carbonyl group in Figs. 12.2.5.2, 3.

1796

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

Neighbour anisotropic diamagnetic shielding region denoted by + sign (cone)

H

er

e

t.m

e/

th e

al

ch

em

yl ib

ra ry

The nature of neighbour anisotropic effect depends on the orientation of the molecule/group containing the 1t-electron cloud with respect to the direction of the field. It is illustrated for some systems. • Ethylene: In an externally applied magnetic field, ethylene is preferably oriented in a way so that plane of the double bond is perpendicular to the direction of the field. Thus the induced circulation of the 1t-electron cloud produces a diamagnetic field (i.e. opposing the applied field) around the C-centres and paramagnetic field in the region of the protons, i.e. C-centres experience the diamagnetic shielding while the protons experience the paramagnetic deshielding from the anisotropy caused by the induced circulation of the 1t-electron cloud of ethylene. This is why, the ethylene protons experience the higher field and resonate at downfield, i. e. resonance at the lower applied field. It gives the higher ~-value. • Acetylene: In an external magnetic field, acetylene is preferably oriented parallel to the direction of the field (i.e. the triple bond is parallel to the field direction). In this orientation, the 1t-electron cloud of the C == C bond is most free to circulate around the molecular axis under the influence of the external field. The magnetic field generated from the induced circulati,on of the cylindrical1t-electron cloud of the C == C bond opposes the applied field in the regions of the protons. Thus these protons experience a lower field due to this dimagnetic shielding from the anisotropic effect. This is why, the acetylinic protons resonate at the higher applied field (i.e. lower 8-value). The horizontal orientation of the molecule (i.e. perpendicular to the direction of the field) experiences a hindered circulation of the 1t-electron cloud causing the paramagnetic anisotropy towards the centre of the triple bond and dimagnetic shielding or diamagnetic anisotropy towards the protons. Thus both the vertical and horizontal orientations of the acetylene molecule provide the shielding at the proton. However, the horizontal orientation of the acetylene molecule is less favoured.

c

(-) III

H

C

lic

k

(-) 'O==C,':;, (-)

Neighbour anisotropic paramagnetic deshielding region denoted by - sign (outside the cone)

Fig. 12.2.5.3 Neighbour anisotropic effect. Conical shaded region (denoted by + sign) represents the anisotropic diamagnetic shielding region while the region (denoted by - sign) outside the conical region denotes the anisotropic paramagnetic deshielding region.

1797

SPECTROSCOPIC METHODS AND OTHER PHYSICAL METHODS IN CHEMISTRY

fj 2

H

al

CH

H

H

2

e/

/

~

t.m

. (6n-electrons) Relatively downfield proton (o~ 2.0 ppm)

H

H

H

H

Annulene [18], (18 n-electrons)

Ring proton (deshielded) (Downfield), 0 ~ 8.5 ppm. CH 3 protons (shielded) (Upfield), 0 ~ 4 ppm.

C

lic

k

H

er

e

Anisotropic paramagnetic deshielding

CH

(

H

H

th e

CH~r2

H

ch

em

Anisotropic diamagnetic shielding Upfleld proton \ (0 ~ -1.0 ppm) ' "

~

Outer protons (downfield) (deshielded) (0 =9 ppm

yl ib

Inner protons (shielded) (upfield) ( (0=-2 ppm)

ra ry

• Benzene: In presence of an external magnetic field, the benzene molecule lies preferably in a plane perpendicular to the direction of the field. The induced circular motion of the 1t-electron cloud around the ring produces a ring current. This induced ring current produces a diamagnetic field at the centre of the ring and a paramagnetic field outside the ring. Thus the aromatic protons at the periphery of the benzene ring expeiience the paramagnetic anisotropic effect, i.e. paramagnetic deshielding. This is why, the aromatic protons experience the higher magnetic field and they resonate at the lower applied field (i.e. downtield resonance and consequently the higher chemical shift cS). In different aromatic systems with (4n + 2)1t electrons, because of the induced ring current (under the influence of a magnetic field), the protons lying outside the ring are deshielded (i.e. paramagnetic anisotropy and higher cS-value) while the protons residing within the ring are shielded (i.e. diamagnetic anisotropy and lower cS-value) (Fig. 12.2.5.4).

Trans-15, 16, dimethyl15, 16-dihydropyrene.

Fig. 12.2.5.4 Neighbour anisotropic effect. Protons lying in the shielding region are the upfield protons and protons lying in the deshielding regions are the downfield protons.

• Carbonyl group: The anisotropic effect indicates that the eRO proton lies in the paramagnetic deshielding region and it shows the higher chemical shift. The carbonyl or nitro group (showing the electron deficiency at the 0- and p- positions due to the electron withdrawing mesomeric effect) in the aromatic ring shows this anisotropic effect for the ortho- protons (i. e. deshielding due to the anisotropic effect arising from the n-electron cloud of the double bond present in the carbonyl or nitro group). The shielding and deshielding regions due to the different types of 1t-electron systems have the characteristic shapes and directions (Fig. 12.2.5.3). Protons falling within the conical region are

1798

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

experiencing the anisotropic diamagnetic shielding while protons lying outside the conical region are experiencing the anisotropic paramagnetic deshielding.

(D) Chemical shift values for C2H 6, C2H4, C2H 2, C6H6 and RCHO: H

R H

HC==CH

-0.9

8 (ppm):

5.3

H H

H

H/

9-10

7.3

2.3

~ C=Q

H

er

e

t.m

e/

th e

al

ch

em

yl ib

ra ry

In terms of protonic character and local diamagnetic effect (controlled by the electronegativity of C-centre), among C2H6 , C2H4 , and C 2H2 , the acetylinic hydrogen is most deshielded and it should show the highest 8-value. The experimental values of 8 can be explained by considering the predominant neighbouring group anisotropic effect generated in C 2H4 and C 2H2 • For C2H 4, both the local diamagnetic effect (controlled by Sp2_C) and remote anisotropic paramagnetic deshielding effect work in the same direction but in C2H2, the local diamagnetic effect and remote anisotropic diamagnetic effect experienced by the protons work in the opposite directions (cf Figs. 12.2.5.2-3). This remote anisotropic effect is more important than the local diamagnetic effect. • C 2H 4 vs. C 6H 6 : In terms of the local diamagnetic effect (controlled by the Sp2_C), the chemical shifts of the protons should be more or less comparable in C 2H4 and C6H6 . But in reality, the C6H6 protons are more deshielded. Though in both C 2H4 and C 6H 6, the protons reside in the paramagnetic deshielding region (remote anisotropic effect), the effect due to the ring current of C~6 is more pronounced. This is why, the C6H6 protons resonate at the much lower field. • C2H 4 vs. RCHO: In both the cases, sp2_C is present and the protons reside in the paramagnetic deshielding region (remote anisotropic effect). The more downfield resonance (8 = 9 - 10 ppm) for the aldehydic proton is due to the better remote anisotropic effect and the electron withdrawing inductive effect (due to the high electronegativity of oxygen) causing a strong local diamagnetic effect to reduce the shielding.

C

lic

k

(E) Chemical shifts for HCI vs. H-CH3 : The chemical shift parameters (8) indicate that the proton in HCI (gas phase) is more shielded than those in CH4• In terms of the protonic character (electronegativity effect) and local diamagnetic effect, HCI proton should be more deshielded. It needs an explanation to rationalise the observation. The linear H-CI molecule (having the cylindrical symmetry) can experience the different types of remote anisotropic effect (arising from the diamagnetic circulation of the electron cloud on the remote atom CI) for different orientations of the HCI molecule with respect to the applied field direction. If the H-CI molecule is oriented parallel to the field direction, the diamagnetic circulation of the electron cloud on the remote atom CI will produce a field that will shield (i. e. remote anisotropic diamagnetic shielding) the proton. But if the molecule is oriented perpendicular to the field direction, the same effect of CI will deshield (t.e. remote anisotropic paramagnetic deshielding) the proton. The relative importance of these two effects on H of H-CI will depend on the relative susceptibilities of CI for the parallel (XII) and perpendicular (Xl.) orientations of HCl. Now let us consider the relative importance of XII and Xl- for H-X (X =CI, CH 3). For the tetrahedral molecule (i.e. X =CH 3) where the C-centre has the spherical symmetry, XII:::: Xl-, the net effect arising from the remote anisotropic effect is zero. For the axially symmetrical molecule or cylindrical molecule (i.e. X = CI), the electron circulation is less for the perpendicular orientation of H-CI, i.e. the remote

1799

SPECTROSCOPIC METHODS AND OTHER PHYSICAL METHODS IN CHEMISTRY

anisotropic diamagnetic shielding effect on H from the parallel orientation of H-CI is more important. This remote anisotropic effect to shield the Hel proton is more important than the local diamagnetic effect which is related with the electronegativity of X (i.e. attached atom). For CH4 (i.e. H-CH 3), the local diamagnetic effect controls the magnitude of shielding while for HCI, the remote anisotropic shielding effect (from the parallel orientation of the molecule) is the main contributing factor. It explains the chemical shift values of the protons of HCI and CH4 .

Deshielding region Deshielding ~region

ch

em

yl ib

ra ry

C.

th e

al

Parallel orientation

(Perpendicular orientation)

e/

Fig. 12.2.5.5 Contribution of neighbour anisotropic effect for the parallel and perpendicular orientations of HCI molecule from the field induced diamagnetic circulation of electron cloud on the remote atom Cl.

t.m

(F) Chemical shifts of R-CH3 vs. Ar-CH3 : Let us consider the chemical shifts of CH3 protons in

C

lic

k

H

er

e

CH 3- CH 3 and C 6H s- CH 3 (i.e. tolune). For toluene, the aromatic protons resonate at the relatively lower field (8 = 7.2, remote paramagnetic anisotropic deshielding effect) while the CH 3-protons resonate at upfield (8 = 2.3). Here it is worth noting that in toluene, the methyl group protons resonate a little downfield compared to the ordinary alkyl protons (cf R-CH 3 , 8 : : : 0.9). It is due to the fact that the methyl protons of toluene are somewhat deshielded by the ring current produced by the 1t-electrons of the attached benzene ring (cf this remote anisotropic paramagnetic deshielding effect is more pronounced for the protons directly attached to the benzene ring). Table 12.2.5.1 Chemical shifts for protons (i.e. PMR signals) in different chemical environments (8 =0 ppm for TMS). ~ (ppm)

'l'

Primary (R-CH3)

0.9

9.1

Secondary (R 2CH2)

1.3

Tertiary (R3CH) Vinylic or ethylenic

Type ofproton

(=CHR)

~ (ppm)

'r

Alcoholic (R-OH)

1-5

9-5

8.7

Phenolic (Ar-OH)

4-8

6-2

1.5

8.5

Aldehydic [ R -

9-10

1-0

5.3

4.7

Carboxylic (R-C0 2H)

10.5-12

-0.5-(-2)

3-5

7-5

Type ofproton

c(

Amino (Ar-NH 2)

:J

(Contd.)

1800

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

Table 12.2.5.1 (Contd.)

S (ppm)

l'

1.7

8.3

Amino (R-NH2)

Acetylinic (==CH)

2-3

8-7

Carbonyl compound

Phenyl (Ph-H)

7.3

2.7

[~C_C~O] / I '\

. ( ,/C=r-r-H I Allyhc

J

Type of proton

H

[~ ~

-C-OR

Alkyl chloride [ Cl

C- H

J

~ C - HJ

6.3

5.2

4.8

Sulfuric acid (H2SO4)

11.5

-1.5

Chloroform (CI 3CH)

7.5

2.5

yl ib

6

8-7

3.7

em

4

2-3

Ester (CH3O-COR)

ch

[F ~

8-7

9-5

6.5

al

Alkyl fluoride

2-3

]

1-5

3.5

Ether

Water (H2O)

th e

Ar-~- H J

4-1.5

3.5

a (ppm): Methyl protons (0.9 -

e/

Benzylic (

6-8.5

l'

6.5

t.m

Aromatic (Ar-H) (in general)

S (ppm)

ra ry

Type of proton

H

er

e

3.8); Methylene protons (1.5 - 3.6); Methine protons (3 - 4.5); Olefinic protons (5.0 - 6.5); Acetylinic protons (2.0 - 3.2); Aromatic protons (6.0 - 8.5); Aldehydic protons (7.8 - 9.9). These are the approximate ranges of O-values for different types of protons.

C

lic

k

12.2.6 Chemical Shifts for the PMR Signals in Metal Hydrides and Organometallics ',see Example 11 of Sec. 12.2.19) In the transition metal hydrides, the PMR signal due to the metal bound hydrogen very often shows the negative values of chemical shift (0 = -5 to --15 ppm). The negative value of 0 indicates that the concerned nucleus is more shielded (relative to that of the reference TMS protons), i.e. the NMR transition in the upfield side of the reference (TMS). For the metal hydrides e.g. [HMn(CO)s], [HCO(CO)4], [HRh(CN)s] 3-, etc. (where the transition metal centres are in low oxidation states), the metal bound protons resonate 5 to 15 ppm to the high field side of TMS (i.e. 0 = -5 to -15 ppm). It is easy to detect the M-H bond by following this characteristic PMR signal because very few other protons appear in this range. This is a diagonostic feature of the transition metal hydrides. In some cases, it may occur even 60 ppm upfield from the TMS reference. The heavy shielding of the metal bound hydrogen arises from the partly filled d-orbital electrons of the transition metals (in low oxidation states). However, the high electron density (i.e. local diamagnetic shielding) around the H-atom bound to the transition metal centres cannot alone explain the so upfield (i.e. too negative O-value) PMR signal. Negative O-value for the carbonyl hydrides like [HCO(CO)4], [H2Fe(CO)4] showing the Bronsted acidity is really unusual in

1801

SPECTROSCOPIC METHODS AND OTHER PHYSICAL METHODS IN CHEMISTRY

terms of local diamagnetic shielding. Some authors have argued the paramagnetic contribution from the nearby transition metal centres to explain the so upfield PMR signals of the hydride ligands bound to the transition metals. This paramagnetic shielding occurs by the incompletely filled d-electron subshell of the transition metal centre which may be itself diamagnetic also. This aspect has been discussed in Examples 11 and 26 of Sec. 12.2.19.

=!

for Rh), the PMR signal due to the Rh-H group shows 8 = -10.6 ppm 2 (i.e. resonance occurs at 10.6 ppm on the high field side of the TMS reference). Actually, the PMR For [HRh(CN)s]3- (I

signal appears here as a doublet one due to the splitting by Rh (I =

!) and centre for the doublet

[W(CH3)6]

8 (ppm):

[Ni(112-C2H4)3] 3.0

1.80

[Cr(11 6- C6H 6)2] 4.10

[Fe(l1s-CsHs)2] 4.0

em

Complex:

yl ib

ra ry

2 corresponds to 8 = -10.6 ppm. This observation strongly supports the Rh-H bond. Chemical shifts for the PMR signals of some representative organometallic compounds are given below.

ch

(cf. 8 = 0.9 for R-CH 3; 8 = 5.3 for C2H4 ; 8 = 5.4 for CsHs; 8 = 7.3 for C6H6)

al

Chemical Shift for Different Types of Hydrides: (AH x)

th e

a(ppm): + 10 (strongly acidic proton);- 0 (H is partially negatively charged); highly negative

e/

(-10 to - 20 ppm) (organometallic hydride complexes)

t.m

12.2.7 Area of the NMR Peak and Number of Nuclei: Integration Principle

H

er

e

Generally area of an NMR peak is proportional to the number of equivalent nuclei for which the peak appears. This integration principle is widely used for the PMR signals. However, this principle is not applicable for the 13C-NMR spectroscopy because the relaxation times of the different 13C-centres vary widely (cf. Sec. 12.2.3)

lic

k

12.2.8 Spin-Spin Coupling and Fine Structure of the NMR Signals

C

(A) A-X molecule where 1=

1

2"

for A and 1= 1 for X: For A, 1=

1

2"

and m/ =

1

±2"; for X, 1= 1 and

= ±1, O. The nuclear magnetic moments of the nuclei are mutually interacting. Let us first consider the effect of A on X assuming the A-X molecule to lie perpendicular to the direction of the field. There are two

m/

possible orientations ( mJ

= ± ~) of A in the magnetic field.

(i) m J = +.!. of A: For this orientation of the nuclear spin, A creates a magnetic field which opposes 2 the applied magnetic field (Ho) at X. Thus the actual field experienced (Hex) by X is less than H o. Hex (by X) = Ho - HA , H A =the field generated by A on X. Thus the nucleus X will resonate at an upper field. (ii) m J = -

~ of A : For this orientation of the nuclear spin of A, it creates a magnetic field which acts

in the direction of the field at X. Thus the actual field experienced by X is greater than H o.

1802

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

= the field generated by A on X.

/

,,--.........

/

I

' \ 1//

ml

1

=+"2

t~

,/',

",

/

\,

t

I

~

\

l

\

Ho

o

\

1

\ ex

o

0 - - - q ) H =H +H

\

H

ragnetic flux

",

/'

'VI

;

\

\""+-/r,,",,,-_/I r

Lines of __,

/ 11 I m =-"2

t 0---0 \

~

__',*,

,

t

/

\"'·-/r:"'--~/ r

Ho

A

ra ry

~--""""', ,/

Thus the nucleus X will resonate at a

H

o

em

0

yl ib

= Ho + HA , HA lower applied field.

Hex (by X)

Fig. 12.2.8.1 Spin-spin coupling between the nuclei (A and X in AX system) having I

=!

ch

for the nucleus A. 2 Note: Here it is illustrated for the perpendicular orientation (with respect to the direction of H o) of the molecule

=_! 2

2

HA (for

of A). The actual mechanism of spin-spin coupling is discussed in Sec. 12.2.9.

e/

m/

th e

al

A-X. For the parallel orientation of A-X, Hex (by X) = Ho +HA (for m/ = +! of A) and Hex (by X) = Ho -

t.m

It is revealed that for two different orientations of the nuclear spins of A, the nucleus X will give two NMR signals and these two peaks will be of equal intensity. The equal intensity of the two peaks occurs m/

e

due to the fact that the two orientations of A (Le.

= ±! ) are of equal probability, i.e. X can experience 2

H

er

the field H o + H A or H o - H A with the equal probability. (lj

c::

i

lic

a::

1 JOH-+I

1+-

(lj

c::

1 1 J HO I

1 1 J OH I

0)

"00 0

1+-1JHO~1JHO-+l

:cco a::

C

0

:cco

k

0)

"00

i ~

(a) Doublet peak for deuterium resonance in HO (splitting by 1H)

•

Ho

Ho

(b) Triplet peak for proton resonance in HO (splitting by 2 0 )

Fig. 12.2.8.2 Expected peaks of NMR signals for HD

(lJ HO

= IJOH )

Note: For a particular operating frequency, the required field for the resonance of deuterium is much higher than that required for proton (i.e. Ho for D » Ho for H; cf. YN(D) = 41.1, YN(H) = 268).

Similarly, the NMR signal due to A undergoes splitting into three components by X for its three possible orientations (m] = ± 1, 0) in the magnetic field. In fact, for the three different spin orientations (m I =± 1, 0) of the nucleus X, in a magnetic field, the nucleus A experiences three different fields,

1803

SPECTROSCOPIC METHODS AND OTHER PHYSICAL METHODS IN CHEMISTRY

H o + H x, H o and H o - H x, i.e. A can resonate at three different fields at a fixed operating frequency. This 1 is illustrated for H-D (I = 2" for H, 1= 1 for D) in Fig. 12.2.8.2.

The peak separation is measured by the spin-spin coupling constant (J). It is: JXA (signal of X is split by A) and JAX (signal of A is split by X). 1 1 H-D: For the HD molecule eH: 1= -, m[ = ±-; D, i.e. 2H: 1= 1, m[ = ± 1,0), the NMR signal .22 due to H is split by D into three components and the NMR signal due to D is split by H into two components (Fig. 12.2.8.2).

H(1 =±' m/ = ±±) and F(1 =±' m/ = ±±). Thus the NMR signal due to 19

ra ry

H-F: l

19F

into two components by lH. Similarly, the PMR signal of lH is split into two components by

is split 19p.

th e

al

ch

em

yl ib

(B) CH3CH20H where CH3CH2 represents the A3 X2 system and CH30H: In CH 3CH 20H, there are three types of protons and three PMR signals (3:2: 1 in terms of the peak areas) are expected (Fig. 12.2.8.3a). It happens so under low resolution but under high resolution, the peak due to the methyl protons is split into 3 components (with the intensity ratio, 1:2: 1) while the peak due to the methylene group is split into 4 components (with the intensity ratio, 1:3:3: 1) (Fig. 12.2.8.3b). This can be explained by considering the spin-spin coupling between the CH3 and CH 2 protons (i.e. spin-spin coupling in the A3X2 system where 1=1/2 for both A and X). Let us first consider the fate of the PMR signal of the Me-protons under the influence of the two methylene protons (neighbouring protons). The different combinations of the two equivalent methylene protons produce M[ = 'Lm[ = ±1, 0 as shown below:

1~

e/ ~~

~~

t.m

11

e

1

= M[

+1

0 -1

(parallel spins, 1 spin state) (antiparallel spins, 2 spin states) (parallel spins, 1 spin state).

er

Effect of the CH 2 protons on the CH 3 protons

'Lm[

C

lic

k

H

• Thus for the three different spin orientations (M[ = ± 1, 0) of the CH2 -protons, the CH3 -protons experience three different fields. • The three possible combinations of "Lm[ can split the PMR signal of the CH3 group into 3 fine components of the intensity ratio 1 : 2 : 1 In the same way, the three equivalent methyl protons can split the PMR signal of the two equivalent methylene protons. The three equivalent methyl protons can combine to produce to 3 1 M[ = 'Lm[ = ±- and ± - as shown below. 2 2 'Lm[ = M[ 3 2 1 +2 1 -2 3 -2

+-

111 Effect of the CH 3 protons on the CH 2 protons

11 ~

1~~

~~~

~~ ~

~~~

1~~

~~~

(1 spin state) (3 spin states) (3 spin states) (1 spin state)

1804

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

For the/our different spin orientations (M[

=±%,±~)

of the CHrProtons, the CH2-protons

experience four different fields. Thus the CH 3- protons split the PMR signal of the CH2- protons into 4 fine components having the intensity ratio 1:3:3: 1 OH (m l = -~) CH 2 (m l =

-~)

c

f~~ \~i:~:i!:; ~:.:~:.

~E=

hv

v = 60 Mhz

(ml=+~)

·····················CH 3

CH2(ml=+~) OH (ml=+~)

t.m

e/

th e

al

ch

W

~##

yl ib

>-

e> Q)

,

em

r

ra ry

......................... CH 3 (ml=-~)

er

e

- - - - - - - - - - - - - - - - - - - - - - -...... ~ Ho OH proton CH 2proton CH 3 proton

H

Fig. 12.2.8.3 (a) Schematic representation of variation of the potential energy levels of OH, CH2 and CH3 group protons of CH3CH20H in the presence of a magnetic field (Ho) (Low resolution NMR spectrum of CH3CH20H).

C

lic

k

Here it is worth mentioning that the equivalent nuclei do not split each other. Thus for the -CH3 group, the PMR signal due to a proton cannot be split by the remaining two protons. • Fate of the OH proton: On the basis of the above argument, the protons of the CH2 - group should split the PMR signal of the OH- group proton into a triplet one. Similarly, the OH proton should split the four PMR signals of the CH2 group into eight components. However, these predictions are not reflected in the PMR spectrum of ethanol (Fig. 12.2.8.3b). Rather, for the OH group proton (which is shielded minimum, cf local diamagnetic effect and electronegativity effect of oxygen), one unsplit PMR signal appears and the PMR signal of the CH 2 group is a quartet one (i.e. splitting by the CH3 protons but no splitting effect by the OH group proton). The observations can be explained by considering the rapid exchange of the OH proton among the different ethanol molecules in presence of a trace amount of protonic acid which efficiently catalyses the exchange process. This proton exchange process is much faster than the NMR transition. Due to this rapid proton exchange among the different molecules, within a very short time interval, a number of protons with an equal probability of the both spin states

(m[ =±~)

may occupy the O-site of a

particular ethanol molecule. Thus only a time average effect of the two spin states of the proton is

1805

SPECTROSCOPIC METHODS AND OTHER PHYSICAL METHODS IN CHEMISTRY

(Low resolution) (Sample experiencing the rapid OH proton exchange)

CH 3 (8

~

1.2 ppm)

(High resolution)

i ----+

Magnetic field strength (H o)

yl ib

ra ry

(Sample experiencing the rapid OH proton exchange)

em

Fig. 12.2.8.3 (b) PMR spectrum of CH3CH20H.

t.m

e/

th e

al

ch

observed by the methylene protons and the statistical average magnetic effect is zero (i.e. I appears as 0).' This is why, the PMR signal of the CH 2 group does not experience any splitting by the OH proton. The PMR signal of the OH group is a singlet one because each hydroxyl proton spends some time on a large number of ethanol molecules within the NMR transition time and it observes an average effect of the methylene proton nuclear spins of all these molecules and this average effect is statistically zero. In other words, the time spent by a proton in a particular ethanol molecule is too short to allow the spin-spin interaction with the neighbouring CH2-protons. This is why, the PMR signal of the OH proton is not split by the CH 2-protons.

e

Effect of rapid proton exchange among the CH aCH 20H molecules

C

lic

k

H

er

• The CH2-protons in a particular molecule experience the average magnetic effect of the nuclear spins of a large number of hydroxyl protons on the O-atom. This time average magnetic effect is statistically zero. • The each OH-proton experiences an average magnetic effect of the CH 2 proton nuclear spins of a large number of molecules. This average effect is statistically zero. • As the exchange rate increases, the fine structure components of a peak (under the condition of no exchange) gradually collapse to give a broad band and then sharpens to a single line (cf Sec. 12.2.15) for the neucleus experiencing the exchange. • CH30H: Similarly, for CH30H due to the rapid OH group proton exchange process, the CH3 protons give a singlet peak and the OH proton also yield a singlet PMR signal, i.e. the PMR signal of the CH3 protons is not split by the OH proton and PMR signal of the OH proton is not split by the CH3- protons (Fig. 12.2.8.4). • PMR spectrum of methanol in ultrapure condition: In absence of the catalysing species (i.e. protonic acids or bases), the OH group proton exchange process is prevented and the PMR signal of the OH group is split into four components by the CH 3-protons. Similarly, the PMR signal of the CH3-group protons is split into a doublet by the OH proton (cf Fig. 12.2.8.4). • PMR spectrum of ethanol in ultrapure condition: In the ultrapure sample (having no trace of acid or base as an impurity to catalyse the proton exchange process among the OH groups), the PMR

1806

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

signal due to the OH group is split by the CH2 protons into a triplet one and each of the 4 peaks of the CH2 group protons (splitting done by the CH3 group protons) is split into a doublet (i.e. 4 doublets giving total 8 lines for the CH2-protons) by the OH group proton. Sample experiencing rapid OH proton exchange

OH

CH 3

(High resolution) Ultrapure sample experiencing no OH

OH

i Magnetic field strength (H o) .

em

--+

yl ib

ra ry

proton exchange

Fig. 12.2.8.4 PMR spectrum of CH 30H.

e/

I~

t.m

I

th e

al

ch

Note: Here it may be noted that the spin-spin coupling interaction is only important between the adjacent groups (i.e. 3JH,H) and it is practically insignificant for the saturated compounds over more than three bonds (i.e. nJH,H is negligible when n ) 3) (cf. Sec. 12.2.9). Thus there is no mutual splitting effect between the CH3 and OH group protons in ethyl alcohol.

H

3J

»

4J

(cf. Sec. 12.2.9)

e

H - f - f -O -

H

er

H H ~3

J

4

J

k

• PMR spectrum of ethyl alcohol in presence of 0 20: It will lead to C 2HsOD through the rapid

lic

exchange process with the solvent D 20.

C

CH 3CH 20H + D20~CH3CH20D + DOH

The deuterium nucleus (I = 1) is NMR active but its NMR frequency is very small (cf. YN(2H) = 41.1 and YNetH) = 267.5; 9.21 MHz for 2H and 60 MHz for IH in a magnetic field of 1.4 T). It occurs so because the magnetic moment of the deuterium nucleus is much smaller than that of a proton. Consequently, for the operating frequency of 60 MHz, the NMR transition for deuterium will occur at a very large magnetic field (about 9.0 T). This is whY,for C2H sOD, no peak due to deuterium will be observed

in the magnetic field range applied for scanning the spectrum. • Deuterium exchange: Basic catalysts are efficient for the phenols and acids while the acidic catalysts are efficient for the alcohols and amines in the deuterium exchange process. RCOOH + D20~RCOOD + DOH; ArOH + D20~ArOD + DOH; (Basic catalyst) RNH 2 + 2D 20

~ RND 2 +

2DOH; ROH + D 20

~ ROD

+ DOH; (Acidic catalyst)

Deuterium exchange leads to the disappearance of the PMR signal of the exchanged proton.

1807

SPECTROSCOPIC METHODS AND OTHER PHYSICAL METHODS IN CHEMISTRY

(C) Number of fine components generated in a peak by a group of equivalent nuclei (e.g. A2X2, A3X2, ••• systems): If an NMR signal due to A is split by nx equivalent X nuclei of the nuclear spin Ix, then the number of fine components (NA ) is given by:

=2nxlx + 1, for I =!,

NA =nx + 1 which is described as the (n + 1) rule. 2 The relative intensities of the fine components (caused by the n equivalent nuclei of I

NA

=.!.) 2

is

obtained from the coefficients of the terms that appear from the binomial expansion of (r + l)x-I where x = number of fine components in the split peak, r = an undefined variable. For illustration, let us take a triplet.peak where x = 3, the expansion is: (r + 1)2 = r2 + 2r + 1, i.e. the intensity ratio is 1 : 2 : 1. For a quartet peak, x = 4, (r + 1)3 = r3 + 3r2 + 3r + 1, Le. the intensity ratio is 1 : 3 : 3 : 1.

2

will split a peak into n + 1 fine components) is helpful to remember the coefficients of the binomial

yl ib

2"

=.!. , the following Pascal's triangle (when n equivalent nuclei of spin

ra ry

For the n splitting nuclei of I

1

em

expansion.

=0 =1 n =2 n =3 n =4 n =5

2

3 6

10

4

10

(Singlet) (Doublet) (Triplet) (Quartet) (Quintet) (Sextet)

Intensity ratio 1: 1 1: 2 : 1 1: 3 : 3 : 1 1: 4 : 6 : 4 : 1 1 : 5 : 10 : 10 : 5 : 1

5 Note: Sum of any two adjacent elements in Pascal's triangle in a row = the element between them in the row just below (cf triangle constituted by 6, 4 and 10 or 2, 1 and 3). Relative intensities of the said (n + 1) hyperflne lines can also be obtained from: n n! em = m! (n-m.), where m = 0, 1,2, ... , n For example, n = 4 (say), relative intensities of the five lines are as 4C O : 4C 1 : 4C 2 : 4C 3 : 4C4 = 1 :

H

er

e

t.m

e/

5

th e

3 4

al

ch

n n

lic

k

4 : 6 : 4 : 1. This intensity ratio for the splitting nuclei of I

C

family free (see Scheme 12.3.5.1 and Table 12.3.5.2)

=.!..2 can also obtained by con~tructing a

(D) AxMXy system: The NMR signal of M is split by both xA and yX nuclei. In such cases, the number of fine components in the NMR signal of M is given by: NM

= (2xIA + 1)(2ylx + 1), IA and Ix are the spin numbers of A and X respectively.

Let us illustrate for AMX 2 having: where N M

A(I =~), M(I =~) and X(I =~)

= ( 2 x 1x ~ + 1)(2 x 2 x ~ + 1) = 6.

The different possibilities are discussed below. (i) JM- X ) JM - A : Two X and A ( I

= ~)

(I =~) atoms can split the NMR signal of Minto a triplet (2 x 2 x ~ + I)

can split each component of the triplet into a doublet, i. e. a multiplet of 6 fine lines

1808

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

will be generated. Thus the NMR signal of M will give three doublet peaks for the condition JM-X ) JM-A where JM-X denotes the coupling constant for the nuclei of M and X and JM-A denotes the coupling constant for the M and A nuclei. (ii) JM- A ) JM- X : A can split the NMR signal of M into a doublet and each component of the doublet is split into a triplet by two X nuclei. It will give a multiplet consisting of two triplet peaks (i.e.

a multiplet of 6 fine components). (iii) JM- A == JM- X: The M-A and M-X coupling constants are comparable. Then the spectral pattern will be of an intermediate nature of the above two possibilities. J M- A I I

(ij

loll

c::

0)

~ ~10II ~~I

(ij

c::

0)

"00

"00

0

i

ch

i

J M- X ) JM-A

+ - - J M- A

C

A

er H

i

~

k

=0

co cr:

giving rise to a three doublet peaks, i.e. triple doublets.

lic

0)

(b) Splitting of the NMR peak of M by both A and X

e

c::

0

- - + Ho

~

(ij

"00

t7

t.m

e/

(i) Splitting of the NMR peak of M under the condition,

th e

al

---+ Ho (a) Splitting of the NMR peak of M by 2X into a triplet

JM-x) JM-A'

Three doublets

em

=0

co cr:

spectrum of HPF2

yl ib

0

=0

co cr:

31 P-NMR

ra ry

J M- X

---+

Ho (b) Splitting of the NMR peak of M by both A and X

(a) Splitting of the NMR peak of M by A into a doublet

(ii) Splitting of the NMR peak of M under the condition, J M- A ) J M- X giving rise to a two triple peaks.

Fig. 12.2.8.5 Nature of the NMR peak of Min AMX 2 system (A:I

= 1/2, X: I = 1/2

and M: I

= 1/2) depending on the

relative values of JM- A and J M- X

Assuming JA - M ) JA - X ; JA- M ) JM - X , we can have: NMR of A (one pair of triplets), NMR of M (one pair of triplets), NMR of X (one pair of doublets). Let us illustrate the 31p_NMR spectrum of HPF2 will produce a muliplet of (2 x I x

(I =~ for lH, 19 F and 31 p ). The 31p_NMR signal

~ + I) ( 2 x 2 x ~ + I), Le. 6 fine components. Depending on the

1809

SPECTROSCOPIC METHODS AND OTHER PHYSICAL METHODS IN CHEMISTRY

relative values of Jp - H and Jp - F , the 31p NMR spectrum will show two triplet bands (if Jp - H ) Jp - F ) or three doublet bands (if Jp - F ) Jp - H ). In reality, Jp - F ) Jp - H (ca. 1500 Hz vs. 250 Hz) and the 31p-NMR spectrum of HPF2 gives three doublets. HPF2 represents the AMX2 system (cf. Table 12.2.11.1).

ra ry

(E) NMR spectrum of vinyl fluoride (AMXZ system): Here the three protons are magnetically nonequivalent in terms of the JHF coupling constants. The first-order 19F-NMR spectrum may be constructed as shown in Fig. 12.2.8.6.

)

3J HF (trans)

)

3J HF (cis)

H

er

e

t.m

e/

th e

al

ch

em

yl ib

3JHF (VI'C)

lic

k

Fig. 12.2.8.6 Schematic representation of the stick diagram of the first-order 19F-NMR spectrum (4 doublets) of vinyl fluoride.

C

(F) Illustration (Prediction of the number of fine components): Analysis in terms of mainly CHEMICAL EQUIVALENCE of the protons (ignoring their MAGNETIC EQUIVALENCE) (i)

(a)

(b)

(c)

H3C-CH 2 -CH 2 - B r

o(ppm):

(ii)

1.2

1.98

(a)

3.47

(b)

(a)

H3C-CHBr- CH 3

o (ppm): 1.75

4.3 (a)

(iii)

1.75 (b)

H3 C-CHO

o(ppm):

2.2

9.8

(a) Triplet (n = 2); (b) Multiplet (12 lines, nl = 2, n2 = 3, assuming Jab '# J bc ); (C) Triplet (n = 2) - downfield resonance due to the electron deshielding effect of bromine. Peak area ~ a : b : c = 3 : 2 : 2. (cf. Sec. 12.2.10; for the magnetic equivalence) (a) Doublet (n = 1), (b) Multiplet (7 components, n = 3 + 3) (downfield, B = 4.3 ppm, deshielding effect of Br); Peak area ~a:b=6:1

(a) Doublet (n = 1), (b) Quartet (n = 3 ) (downfield due to the anistropic effect of the 1t-electron cloud and electron withdrawing inductive effect of 0); Peak area ~ a : b = 3 : 1

1810

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

(iv)

(c)

(b)

H

H

(c) 7.6

(b) 8.1

(c)

H

8 (ppm):

(a) 2.45

(v)

At 60 MHz (probably 2nd order effect): (a) Singlet, (b) Multiplet (ortho-protons deshielded by the anisotropic effect of the carbonyl group); (c) Multiplet (assuming two m- and one p~ protons equivalent) (ef benzaldehyde at 300 MHz, xix). Here interpretation is simply given in terms of chemical similarity/equivalence of the 5 ring protons. The 5 ring protons actually respresent the ABB'MM' spin system (Sec. 12.2.11).

ra ry

(a) Singlet, (b) Singlet (5 aromatic protons are of three types: 2 'J-H, 2 m-H and 1 p-H with respect to the Me-group; but these aromatic protons are hardly affected by the Me-group)

y (b)

(b) 7.2

(a) 2.3

yl ib

8 (ppm): (c)

8 (ppm): (a) 3.7

(vii)

(b)

(b) 6.85

(c) 7.15

e/

H

(b)

(c)

H

H

(a)

e

CH 3

er

°2

N (b)

(C)H

H

H

k

(b) 6.7

C

lic

8 (ppm): (C) 7.4

Crude analysis of the ring protons in terms of chemical equivalence: (a) Singlet; (b) Doublet (split by the neighboring proton ortho- to the N0 2 group); (c) Doublet (split by the proton ortho- to CH 3 group) (downfield, deshielding effect of the N0 2 group, both electron-withdrawing mesomeric effect and anisotropic effect of the n-bond of the N02 group on the orthohydrogens). Note: In terms of chemical equivalence, the 4 ring protons show (crudely) two doublets (i.e. 4 lines) in the PMR spectroscopy. This aspect is discussed later. In terms of magnetic equivalence, the 4 ring protons actually represent AA'BB' system (Sec. 12.2.10).

t.m

(C)H

al

O-CH 3

th e

H

em

(a)

(b)

(a) Singlet (shifted downfield because of the electronegative O-atom), (b) Multiplet (electron pushing mesomeric effect, 0 and p positions are more shielded and relatively upfield), (c) Multiplet (m-protons relatively downfield) (ef the problem of acetophenone and benzaldehyde).

ch

H

(vi)

(a) 2.4

3F (equivalent, C 3 axis), one by one

31

p

(I =~)

19F(I =~)

NMR signal is split

into a doublet.

The 31P-NMR signal is split into a quartet (n

(x)

CI---@-CI

= 3) by 3F.

12 equivalent protons, i.e. one PMR signal which is split into = 1) (ef 2 x 1 x 1 + 1).

a triplet by 14N(I

4 aromatic protons are assumed to be equivalent, i.e. one PMR signal.

1811

SPECTROSCOPIC METHODS AND OTHER PHYSICAL METHODS IN CHEMISTRY

(a)

In terms of chemical equivalence, the 4 ring protons should give two 2 PMR signals (each split into a double by the other, i.e. two doublets). In reality, the observed complex multiplet PMR'signal can be explained by considering the magnetic equivalence of the ring protons representing the AA'BB' spin system not the AB spin system. Let us compare the chemical equivalence of the 4 ring protons in the given 0- and p- isomers. The p-compound should give one PMR signal for the 4 equivalent ring protons while the o-compound should give 2 PMR signals (each split into a doublet by the other, i. e. two doublets) for the 4 ring protons. In reality, the p-isomer gives one PMR signal while the o-isomer gives a complex multiplet PMR signal for the ring protons. p-isomer: 8 ~ 7.2 ppm (P = CI); 8 = 7.2 ppm (ring protons) and 2.3 ppm (2 Me protons) for P = Me. Note: Por finer analysis, the magnetic equivalence of the ring protons is to be considered.

c'jQ(.(b (b)

J

(a)

CI

p A

H

A

A

H

ra ry

P

F--@-F

Jet

al

4 aromatic hydrogens are not equivalent in terms of 9 p- 1H) coupling constants; it gives a complex multiplet

th e

(xi)

ch

em

yl ib

HA

PMR signal. (a)

(xii)

(b)

(c)

(a) Triplet (n = 2), (b) Triplet (n exchange of the OH proton)

t.m

e/

NC-CH 2-CH 2-OH:

p A'

H

and

B'

a

H

I I

~cr

B'

H

C

H

lic

B

k

H

H

er

e

(xiii)

(ct. Sec. 12.2.10)

A'

= 2),

(c) Singlet (rapid

In terms of magnetic equivalence, the 4 ring protons actually represent the AA'BB' systems (to be discussed later) and the PMR signal is a complex one. Thus o-dichlorobenzene gives a complex PMR spectrum. These aspects will be discussed later in Sec. 12.2.10. Note: H A and H A ' are chemically equivalent. Similarly, HB and H B' are chemically equivalent. Both sides (with respect to the a-plane) of the ring are identical and consequently

both sides of the ring show the identical splitting in the PMR spectroscopy. The PMR signal of HA is split into a doublet by H B and similarly the PMR signal of H B is split into a doublet by H A • It gives two doublets for H A and H B • Similarly, H A' and H B' give two identical doublets. Thus the 4 ring protons produce (crudely) two doublets (i.e. 4 lines) in the PMR spectra. Thus considering the same chemical shifts (i.e. chemical equivalence) of H A and H A ' and similarly the same chemical shifts of H B and H B', we can expect two doublets (i. e. 4 lines) for the 4 ring protons in the crude PMR spectra. This characteristic 4 line PMR spectrum has been crudely observed in many cases for the disubstituted para- and ortho- compounds as shown above. However, under high resolution, they behave as the AA'BB' spin system for the 4 ring protons to produce a complex spectrum (2nd order effect).

1812

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

x M

M'

H

(xiv)

H

It represents the ABB'MM' system for the 5 ring protons (to be discussed in Sec. 12.2.10) and it gives a complex PMR spectrum. Benzaldehyde, acetophenone, etc. also belong to this spin system.

(X = F, Sr, CI) B'

H

ra ry

(ct. Sec. 12.2.10)

Two 19F-NMR signals: one doublet (for the 4 basal F atoms) and one quintet (for the axial F).

yl ib

(xv) BrFs and IFs: (C4v ' square pyramidal)

em

PMR SPECTRA IN TERMS OF CHEMICAL EQUIVALENCE ONLY

al

ch

For the PMR studies, JH,H is not very large·. Consequently, considerating the chemical equivalence of the protons (ignoring their magnetic equivalence), the PMR spectra can be at least crudely interpreted in most of the cases.

II

CH 3-

(b)

C-

(c)

CH 2-CH 3

(a) 2.12

(b) 2.45 (a)

b (-4)

(b)

k

(c)

(a)

°2N-CH2-CH2-CH3 a (-1.0), c (-4.5)

b (-2.0),

C

8 (ppm):

(a) Triplet for the methine protons (downfield; attached to two Cl-atoms); (b) Doublet

H

a (-5.8),

lic

(xviii)

er

CI-CH 2-CHCI 2 8 (ppm):

(c) 1.05

e

(b)

(xvii)

t.m

(Methyl ethyl ketone) 8 (ppm):

(a) Singlet (attached to the CO group relatively downfield), (b) Quartet (n = 3), (relatively downfield, effect of the adjacent CO group), (c) Triplet (n = 2) (relatively upfield).

e/

(a)

th e

o

(xvi)

(xix)

b

H

(cf. acetophenone, iv) 8 (ppm): 9.9 (CHO proton), 8.1 (o-protons), 7.5 (m- and p-protons)

(a) Triplet; (b) Multiplet: 6 lines (n = 5) (if Jab =J bc ); 12 lines (nl = 3, n2 = 2) (if Jab '* J bc); (C) Triplet (downfield, effect of the adjacentN02 group). Note: At 60 MHz, H(b) - protons show 6 fine lines, i.e. splitting by 5 protons, i.e. n =5 (3 H(a) + 2 H(b». It supports, Jab::::: J bc • In other words, overlapping of some lines of the 12 lines (splitting by 3H(a) and 2H(c), Jab "# Jbc ) give the 6 lines. Interpretation in terms of chemical equivalence of the 5 ring protons: At 60 MHz (probably 2nd order effect), 2 PMR signals (multiplets) for the ring protons; 2 orthoprotons (downfield); 2 meta-protons + 1 para-proton (relatively upfield). At 300 MHz, 1st order effect and n+l rule can explain the findings: He (doublet, splitting by Ha); Ha (triplet, splitting by Hb and HC); Hb (triplet,

1813

SPECTROSCOPIC METHODS AND OTHER PHYSICAL METHODS IN CHEMISTRY

splitting by two Haprotons); HC (downfield, anisotropic effect of the CHO group); between Ha and Hb, Hb resonates slightly downfield (resonance effect). Note: Two chemically equivalent Ha protons are magnetically nonequivalent; similarly two HCprotons are also magnetically nonequivalent. Actually, the 5 ring protons represent the AA'BB'C system under high resolution.

12.2.9 Spin-Spin Coupling and Coupling Constant (J)

em

yl ib

ra ry

The degree of coupling interaction (causing splitting of the NMR signal) between the nonequivalent nuclei is measured by the coupling constant (J) which is expressed in the unit Hertz (Hz). • Mechanism of Coupling: There are many theoretical models to explain the mechanism of coupling. According to the Dirac Model, the electrons of the intervening bonds carry the spin information between the interacting nuclei through the interaction between the nuclear spin and electron spin. An antiparallel arrangement of the nuclear spin and electron spin stabilises the system more than their parallel arrangement. In this antiparallel arrangement of the electronic and nuclear spins, the electronic magnetic dipole is opposed to that of its own nucleus. The l3C_1H coupling may be understood as follows: e

m/

== +.!.. of l3C, the bonding electron with spin 2

~ Antiparallel

nuclear spins i.e. positive J-value

(Higher energy)

t.m

For

t

e/

(Lower energy)

_.!.. 2

er

e

found near l3C. Obviously, the other bonding electron of to adopt the nuclear magnetic quantum number

H

'H

al

t

th e

t~

'3C

e

t+--+-t

ch

~ Nuclear spin ~ {Electron spin

Antiparallel nuclear spins

will have the highest probability of being

+! 2

spin residing close to IH will direct IH

_.!.. (i.e.

m/ == _.!.. for IH). Similarly, for m/ == _.!.. of 2 2 2

2

lic

k

l3C, the bonding electrons will direct IH to have m/ == +.!... In this indirect spin-spin interaction, i.e.

C

electron coupled spin-spin interaction leading to nucleus-electron-electron-nucleus with the alternate spins, when the proton spin interacts with the l3C-spin, it 'sees' two different orientations of the l3Cspin. Thus two different nuclear spin states of t3C give two different NMR transitions of tH. In terms of

m/

=

±"21

of IH, similarly two NMR transitions of l3C can be understood.

• Field independent parameter (J): It may be recalled that the direct spin-spin coupling effect as illustrated in Fig. 12.2.8.1 depends on the orientation of the molecule (A-X) with respect to the field. Considering all the possible orientations of the molecule (A-X), the statistical average coupling effect becomes zero, i. e. all the orientations leading A to reinforce the field at X are just balanced by the other orientations reducing the field at X. The electron coupled indirect mechanism of spin-spin coupling indicates that it mainly depends on the nature of the intervening bonds between the nuclei but not on orientation ofthe molecule in the field. This is why, J is independent of the field strength but the chemical shift, i.e. frequency shift (Av in Hz) from the reference depends on the field strength. If there are two peaks for the NMR signal then it can happen due to: two nonequivalent nuclei (i.e. two different chemical shifts) or spin-spin coupling arising from a nearby nonequivalent nucleus with

1814

I

1

= "2.

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

If the peak separation increases with the increase of the field strength then it indicates that the

two peaks are for two different nonequivalent nuclei. If the peak separation does not change with the field strength, then the peaks are due to the spin-spin coupling. • Sign of J: Dirac's model suggests that if the number of intervening bonds between the interacting nuclei is odd (e.g. IJ,3J, ... ), then J becomes positive. If the number of intervening bonds is even (e.g. 2J, 4J, ... ), then J becomes negative. There is no difference in the spectral appearance of the fine components for the negative and positive J-values but with the change of the sign of J, the .individual lines of a multiplet are only interchanged, i.e. upfield and downfield fine components are inter-

ra ry

changed. • Homonuclear couplings: JeH-IH), J( 13C_ 13 C), J( 3I p_3I p), etc. • Heteronuclear couplings: J( 13C- IH), Je 4N- IH), Je 9F- IH), J(3Ip- IH) etc. • Magnitude of the coupling constant: The magnitude of J depends on the number and nature of

em

yl ib

the intervening bonds and the spatial relations between the interacting nuclei. The value of J decreases with the increase of distance. Generally, one bond coupling (i.e. IJ) is stronger than two bond coupling (2J) which in tum is much larger than three bond coupling (3J), i.e. IJ ) 2J ) 3J ).... The coupling beyond the three bond distances is insignificant but such long range couplings may be important in the

1H

(Hz): 125 ( sp 3_C ) < 170(sp 2-C ) < 250(sp-C)

al

J 13C _

ch

unsaturated compounds.

lic

k

H

er

e

t.m

e/

th e

It indicates that the greater s-character in the C-H bond gives the better spin-spin coupling interaction (cf s-orbital electron has a finite probability at the nucleus while the p, d,forbital electrons have the zero-probability at the nucleus). Coupling constants (absolute values) of some systems are given below.

C

3J.

5J (para)

CIS

=3 -

18 Hz

< 3

I

..Jtrans

= 12 -

24 Hz '

(ortho) = 6 - 10Hz; 4J am (meta) = 2 - 3 Hz; 5J ap (para) with the increase of distance, J-decrease.

3 Jao

=0 -

1 Hz;

1815

SPECTROSCOPIC METHODS AND OTHER PHYSICAL METHODS IN CHEMISTRY

Mil = Pd", Pt"; 2Jtrans (31 p_31 P) ) 2J cis (31 p_31 P) 1J195Pt_31p

(cis) > 1J195Pt_31p (trans)

th e

al

ch

em

yl ib

ra ry

Note: Splitting of the PMR signal for the N-H group by 14N(I = 1) is not generally resolved and it gives a broad signal. The fast quadrupole relaxation (T1is very short) for the system is responsible for this. • Karplus relationship: Vicinal coupling constant (3JH,H) depends on the dihedral angle (a) and it is minimum for the gauche conformation (a =90°) and maximum (3JH H::::: 10-12 Hz) for the eclipsed conformation (a = 0°) and anti-, i.e. trans- conformation (a = 180°). The variation of 3J with the dihedral angle (a) is mathematically expressed by Karplus equation. For a = 90°, the two adjacent C-H bonds are orthogonal and at this condition, there is a minimum orbital overlap interaction between the C-H bonds to carry the spin information. It makes J very small for a = O. For a = 0 (i.e. parallel orientation of the C-H bonds) and a = 180° (antiparallel orientation of the C-H bonds), the good overlap interaction makes J larger. H

~H

12.0

t.m

)6{~ 6.0

:I:

3 JHtH

k

I

lic

~

4.0

,,2-4Hz

(anti)

10-12 Hz

H

C

t

$~

H

N

C')J

H~3JHtH (gauche)

er

8.0

H trans or anti

e

eclipsed

~

e/

rh

2.0

Fig. 12.2.9.1 Approximate variation of 3JH ,H with the dihedral angle ex (Karplus relationship).

• [PtX2(PR3)(PR3']: This square planar complex can have the cis- and trans- isomers.

2J p _

p

(trans)2 J p _

p

(cis) ;

IJ195Pt_31p

(Cis)1 J195Pt_31p (trans).

These coupling constants can help us to identify the cis- and trans- isomers. • RnPFs- n (trigonal bipyramidal): The coupling constant for the equatorial fluorine is higher, i.e. I

J 31 p_19 F (eq) ) 1J 31 p_19 F (ax)·

1816

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

12.2.10 Magnetically Equivalent and Nonequivalent Nuclei: Magnetic Equivalence vs. Chemical Equivalence .

yl ib

ra ry

• Chemically equivalent nuclei: If the nuclei are related in terms of symmetry operation (plane of symmetry or axis of symmetry), they are called chemically equivalent. If two centres are chemically equivalent then substitution of each centre by some other atom will produce the same or enantiomeric compound (i.e. mirror image). Chemically equivalent centres show the same resonance frequency for the NMR transition (i.e. same chemical shift) and thus they are isochronous. • Magnetically equivalent nuclei: If the chemically equivalent nuclei (related through the symmetry operation) are isochronous (i.e. same chemical shift) and they do not mutually split each other in the NMR spectra, then the such nuclei are both chemically and magnetically equivalent. Thus for the magnetically equivalent nuclei, the coupling interactions (measured by the coupling constant 1) with all other nuclei of the molecule must be the same. Magnetically equivalent nuclei must satisfy the following two conditions. (i) Magnetically equivalent nuclei must be isochronous (i.e. same chemical shifts), i.e. they must be chemically equivalent.

-~C=C:':-::---

--»

Both 1 H ( I

e

" ' - F(b)

er

(b)H/

t.m

e/

th e

al

ch

em

(ii) Magnetically equivalent nuclei must have the same coupling constants (J) with all other nuclei. In other words, the magnetically nuclei do not split each other and they give a single NMR signal. Thus it maybe mentioned that the chemically equivalent nuclei may not be necessarily magnetically equivalent but the reverse is true. (A) Chemically equivalent but not magnetically equivalent nuclei: (a) Let us compare 1,1- difluroethene and difluoromethane. In each case the protons as well as the fluorines are chemically equivalent. Presence of C2 and cry containing the C 2 axis make the F~ c atoms, i.e. F(a) and F(b) chemically equivalent. Similarly, (a)H F(a)( 2 H(a) and H(b) are chemically equivalent.

H

3

H(a) p(b)

= 3 Jtrans

C

3J

lic

k

It implies that F(a) and '# 3 J

F(b)

H(b) p(b)

1) and

="2

19 F (

I

1) are magnetIcally . actIve. .

= "2

J cis = 3J H(a) p(a) '# 3J trans = 3 J H(a) p(b)

are magnetically nonequivalent.

= 3 J cis.

It implies that H(a) and H(b) are magnetically nonequivalent.

In general, in alkenes, 3J trans is larger than 3J cis. Presence of cr containing the CH2 or CF2 in CH2F2 suggests that both hydrogens as well as the fluorines are chemically equivalent. They are also magnetically equivalent I . 2 F(a) d F(b) . II J H(a)p(a) = 2 J H\a)p(b) I.e., /C" an are magnetlca y H(b) F(b) equivalent; 2 J H(a)F(a) = 2 J H(b)F(a). i.e. jfa) and jfb) are F(a) magnetically equivalent. (b) In 2-methylpropene, as in the case of 1, I-difluoroethene, the hydrogens as well as the methyls are chemically equivalent but not magnetically equivalent. H(a)~ 2J

I

H"

/CH

3

-----.C=C----( C

H/

"CH

2 3

1817

SPECTROSCOPIC METHODS AND OTHER PHYSICAL METHODS IN CHEMISTRY

(c) Let us consider the para- and ortho- disubstituted benzene. Presence of cr makes ya) andya') chemically equivalent and also yb) and H(b') chemically equivalent.

P H(a')

3 J H(a)H(b) "# 5 J H(a')H(b)

,

i.e.

ya)

and ya') are not magnetically

equivalent. Thus it represents the for the four ring protons AA'BB' spin system for the four ring protons.

i.e.

Ha)

and Ha') are magnetically non-

ra ry

3 J H(a) H(b) "# 4 J H(b) H(a')

H(a)

~

(a)

th e

J

F

Presence of cr makes the F-atoms

"-':::

H(b)

c;

~

t.m

(a'

chemically

equivalent but they are not magnetically equivalent. 3 J F(a)H(a) "# 4 J H(a)F(a') , i.e. F(a) and F(a') are magnetically

---- ---- ------H

(I = ±)

e/

r 3

al

ch

em

yl ib

equivalent (though chemically equivalent). Similarly, H(b) and yb') are magnetically nonequivalent though chemically equivalent (i.e. AA'BB' spin system) Note: For the disubstituted ortho- and para- compounds having a plane of symmetry, by considering the chemical equivalence of Ha and Ha'; and chemical equivalence of Hb and Hb', the 4 ring protons crudely show two doublets in PMR spectroscopy (see Sec. 12.2.8).

(b')

H

e

nonequivalent but chemically equivalent. In the same way RCa) and HCa') are magnetically nonequivalent but chemically equivalent. (d) Let us consider, CH3-CH2-X (X = halide, alkyl group) where the three CH 3-protons are chemically equivalent and similarly the two methylene protons are chemically equivalent. Now let us examine the magnetic equivalence of the protons. If the rotation about the C-C bond is considered to be fast, then all the three methyl protons have the same time averaged magnetic environment. The coupling constants of the protons of the CH2 group for each of the three methyl

er

F(a')

C

lic

k

H

~

J = 5-12 Hz C

H

9

Hd )

H

C

H

Hd

J=2~HZ

c

H

Hb

H

x

X

X

(I)

(II)

(III)

Fig. 12.2.10.1 Conformations (i.e. 3 rotational isomers) ofCH 3CH2X where the CH 3- protons are labeled as Ha, H b and He. A particular proton, say Ha is anti to X in one conformation (I) and gauche in the other two conformations (II and III) ; JH,H(gauche) :::: 2 - 4 Hz; JH,H(anti) :::: 10 - 12 Hz (cf. Karplus Rule, Sec. 12.2.9).

1818

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

protons become identical only if the three possible structures (I, II, III) undergo rapid interconversion through the rotation around the carbon-carbon single bond (cf. Fig. 12.2.10.1). .If the rotation about the C-C single bond is locked by placing a bulky group for X and lowering the temperature, then the magnetically equivalence will be lost because the coupling constants will be different for the different dihedral angles (cf. Sec. 12.2.9; J HCHd J HbH d , i.e. Hb and He are magnetically nonequivalent in I). . (e) Cyclohexane: If the interconversion between the two chair forms of cyclohexane is very fast (i. e. rapid interconversion between the axial and equatorial hydrogens), then one PMR signal is obtained. If the interconversion rate is slowed down by lowering the temperature or through substitution then two different PMR signals for the nonequivalent axial and equatorial hydrogen will be observed. (f) In XCU 2-CU2X, all the 4 protons are chemically equivalent (existence of a symmetry plane bisecting the C-C bond). If the rotation around the C-C bond is restricted, the protons become magnetically nonequivalent. When the free rotation around the C-C bond is very fast (compared to the NMR time scale), each proton experiences an identical time averaged magnetic environment. The magnetically nonequivalence of the protons is evident from the coupling constants in a particular locked conformation (Fig. 12.2.10.2)

th e

al

ch

em

yl ib

ra ry

*

(gauche)

:I:- JH,H

(anti)

t.m

e/

; JH,H

(II)

(III)

er

e

(I)

k

H

Fig. 12.2.10.2 Interconversion among the three rotational isomers of XCH 2-CH 2X through the C-C bond rotation.

x'"

/X

z/

"'H

C

lic

(g) Similarly, it can be shown that in Y - C - C -Us, the two protons Ha and Hb are magnetically b

/

Cl

nonequivalent. This is why, in H 3 C - CHC1- C-H a . 1ent. nonequlva

"

,

the protons Ha and Hb are magnetically

Hb

(B) Chemically equivalent nuclei (related through the symmetry operation and same chemical shift): For the PMR studies, specially where JH, H constant is not very large, the chemically equivalent protons generally give one signal (which may be split by the other nuclei). However, under high resolution, the magnetically nonequivalence (if it exists) may be noted. Chemically equivalent nuclei in different compounds are illustrated in the following examples. (i) CU 3-X (C3v ): All the three Me-protons are related by the C 3-symmetry operation and 0- symmetry operation. Thus the Me-protons are chemically equivalent. Assuming the free rotation

1819

SPECTROSCOPIC METHODS AND OTHER PHYSICAL METHODS IN CHEMISTRY

about the C-X bond, i.e. C3- axis, all the three Me-protons experience the identical time average magnetic environment. Thus the three Me-protons are also magnetically equivalent and they give one PMR signal. (ii) Examples of chemically equivalent protons are given below: CI

H3

C",

'0-

I

- - -. C===0 - -~ .

- - - - - - - - 0-

Br-CH 2 -:-CH 2 --Br I I

HC/ 3

CI

(4 protons chemically equivalent, one PMR signal) assuming rapid rotation around the C-C bond

(12 protons chemically equivalent, one PMR signal)

yl ib

ra ry

(6 protons chemically equivalent, one PMR signal)

4 ring protons chemically equivalent; one PMR signal 8 = 7.2 ppm.

/H

C=rC H

/

'"

(b)

(a)

H(al

~(bl : CH 3

1

(b)

(c)

CH-CH 3 2 -OH

,

(~

~'-

i\

10-

e/

er

H

k lic

3

two PMR signals.

(a)

CHX- CH 3

/

(b)H C

Two types of protons, H(a) and H(b),

Two types of protons and 2 PMR signals

C

(a)H

(b)

(i centre of symmetry)

e

(a)

CH 3 -

C0 2H

t.m

Two types of protons, H(a)and H(b); two PMR signals (6:4 intensity ratio); 8 = 7.2 ppm (ring protons), 2.3 ppm (Me-protons).

C

th e

C2

CH~b)

(iii)

~\

al

(a)

II

(a)

H02C", \ H(a)

o

em

\

ch

(b)

~)

Two types of protons, (H(a) and H(b)). two PMR signals, peak area ratio 6:2 (a)

(b)

CH 3 -

CH 2

(c) -

CHO

~)

CH 3 - - CH 2 --CH 2 - - X

""'"

V

_-------J

In each case, 3 types of protons ann

3 PMR signals.

Two vinylic protons are not equivalent: H(b) (cis- CH 3 , transX), H(c) (cis- X, trans- CH 3). Three types of protons and 3 PMR signals.

It may be noted that the substitution of the vinylic protons by A produces a pair of diastereomeric products, i.e. vinylic protons are diastereomeric protons (not enantiomeric protons). (iv)

Two BHB bridges lying in a plane perpendicular to the plane of the B-H terminal bonds. Two types of protons and two PMR signals (decoupled from lIB).

1820

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

Quadrupole nuclei Nand CI do not couple with the methyl protons. Two equivalent

II B

(I =%)

nuclei split the PMR

signal of the Me-protons (cf. Me groups are equivalent) into 7 components.

(v)

C2 ,tJ R P------------Br

Br- - - - - - - - -- - - PR

:~/:3

:/Pt

:

1

1

1 "I

",.

Cl-----------PR3 1

ra ry

1

3:~/:

:

,,'

'"

CI -----------PR3 ,,'

(cis-)

yl ib

:/Pt

In the trans-isomer, the two PR3 groups are related through the C 2-axis (CI-Pt-Br) and cry (containing the C 2 axis and perpendicular to the molecular plane). Thus these two chemically equivalent PR3 groups give one 31p_NMR signal which can ·be split into a

doublet through the 1 J 195 pt _31 p coupling

(trans-)

em

1 (cf. 1=- for 195Pt). 2

(vi)

R P------------X

er 2

1

:

'"

1

1

,,'

x------------PR3 ,,'

H

k

lic

:/Pt

,

2

In each case, the two 2PR3 groups are equivalent, i.e. one 31p-NMR signal which will be split by 195pt. The two chemically equivalent PR 3 groups are not always magnetically equivalent and the 31p-NMR signals may be split.

(trans-) D2h

C

C2v

e/

3:~/:

e

3:~/:

(cis-)

(;C

R P------------X'

----:----- Pt-----f ~ :/~: C R P------------X 3

(cis) > IJ195Pt_31p (trans)

t.m

1 J195Pt_31p

th e

al

ch

In the cis-isomer, the two PR3 groups are not related through the symmetry element like en or cr and thus these two PR3 groups are not equivalent and they produce two different 31p NMR signals. Each signal is split by both 195pt and the other nonequivalent 31p nucleus producing a double quartet. The difference in the coupling constants can also distinguish the isomers,

The difference in the coupling constants, IJ pt _ p (cis) ) IJ pt _ p (trans) and 2J 31 p _31 p (trans»> 2 J 31 p_ 31 p (cis) can distinguish the cis- and trans- isomers (cf. the problem of [PdCI 2(PR3)2], Sec.

12.2.19). (vii) All the F-atoms in each of the following molecules are equivalent, i.e. one 19F-NMR signal in each case.,

F-Be-F

(4F related by C 4 )

(3F related by C3 axis)

1821

SPECTROSCOPIC METHODS AND OTHER PHYSICAL METHODS IN CHEMISTRY

v

v (C 3v), 3F related by C3 axis Il BFi ;

one weak septet (weak signal) for

10 BFi

ra ry

one quartet (strong signal) for

BF} 19 F - NMR:pectrum

F

(C

C 2v

)

2

ch

/

IOB(20%, 1=3)

em

..\c=otF

%}

yl ib

(see Fig. 12.2.19.8); llB(80%, 1=

:

/

"" •. ""

th e

(A = Sr, I)

t.m

One quintet (for 1 axial F-atom)

AFs

e/

~~re4d~a~~~~_atoms) !~i7i

al

(viii) Two types of F atoms, i.e. two 19F-NMR signals in each of the following two cases. F

11

/

F........ ~/ .. F"

,"""/A~F : "

•

"" I

\

:

: /" F ... ......

4 basal F + 1 axial F (A4X spin system)

:\

.: ' " ......:... F' F

(A = I, Re)

',

k lic

: Two types of P; two 31P-NMR signals.

C

(ix)

3 equivalent

One signal doublet (split by 1 P) and other signal quartet (split by 3P)

P-centres

CI

I

(b)

F

N-=P-CI

"'.(aV P F

(x)

/

~

'" N

(b~

N--P-CI

I

CI Two types of P-centres; X2 denotes the two equivalent F-centres.

AB 2X2 spin system (complex NMR spectra) for the P-atoms.

One triplet (for 5 basal F-atoms) One sextet (for 2 axial F-atoms)

5 basal F + 2 axial F A SX2 spin system

H

er

e

F------------F

.

F

1822

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

CI (a)

CI

I I N

I I N

(b)

CI-P===N-P-NHCsH s

II

(xi)

> 3Jo, 0). On the other hand, the -CH2CH2- moiety of BrCH 2CH2CI behaves as an A2B2 system in PMR spectroscopy (Fig. 12.2.12.4). The similarity in the electronegativities of Br

1829

SPECTROSCOPIC METHODS AND OTHER PHYSICAL METHODS IN CHEMISTRY

em

yl ib

ra ry

and Clleads to the chemical similarity of the two methylene group protons. It reduces the chemical shift difference between these two sets of protons. But these two sets of protons are magnetically nonequivalent (cf. rotation around the C-C bond and rotational isomerism, Sees. 12.2.10, 12.2.20). For these two methylene group protons, due to the decreased difference of chemical shift, the condition of 2nd order PMR spectra (i.e. Av == 3JH, H) is attained and the -CH2CH2- moiety behaves as an AzBz system in PMR spectroscopy.

i

ch

~

~

BrCH 2CH 2CI

al

~

(~v ~ JH,H)

2nd order spectra

t.m

e/

th e

A 2B 2-system

Chemically almost similar but magnetically nonequivalent

er

e

Fig. 12.2.12.4 Qualitative representation of the PMR spectra (high resolution) of bromoethane and I-bromo2-chloroethane.

H

12.2.13 Chemical Shift Reagents

C

lic

k

In some cases, the PMR signals of the organic compounds overlap and they cannot be resolved. The situation can be simplified by using the higher frequency (e.g. 300 MHz, 400 MHz, etc.) NMR spectrometer but such instruments are costly. Sometimes, change of solvent (which can interact with the sample through the H-bonding to change the position of the PMR signals) may help to simplify the spectra. The use of solvents like benzene instead of CCl4 or CDCl 3 can change the position of the PMR signals by imposing the anisotropic effect on the surrounding moieties of the sample. However, the use of chemical shift reagents can change the positions of the PMR signals (without any peak broadening) and it is an inexpensive technique to resolve the overlapping signals. CMe3

Eu

o~ I

CH

o~

CMe3

[Eu(dpmbl tris-( dipivaloylmethanato)europium(lll)

o Eu

CF CF

... The pyramidal structure of PF3 having three equivalent F-atoms is also supported from the 31p NMR spectrum. The 31p_NMR signal is split into a quartet (intensity ratio 1:3:3:1) due to the

em

SPlitti~g by 3 equivalent F atoms ( 2 x 3 x -i + 1 =4). Here it is worth mentioning that the trigonal

C

lic

k

H

er

e

t.m

e/

th e

al

ch

planar structure of D 3h symmetry (where the 3F atoms are also equivalent) will also give one 31p-NMR signal having four fine components (splitting by 3F atoms) and one 19F-NMR signal split into a doublet by 31p. However, the NMR spectra can distinguish between the C 3v (pyramidal) and C 2v (T-shaped) symmetries of PF3. 7. NF3: It also adopts the pyramidal structure (C 3v ) as in PF3. Because of the rapid nuclear quadruple relaxation of 14N (I = 1) (cf. Sec. 12.2.16), the single 19F-NMR peak is not split by 14N. In fact, at very low temperature (say, -205°C), it gives a single unsplit peak for 19p but with the increase of temperature, the NMR peak starts to broaden at about 20°C and it gives a sharp triplet at the higher temperature, i.e. 19F-NMR peak is split into a triplet peak by 14N (I = 1).

i

_ F NMR spectrum of PF3 19

(One doublet peak; splitting by one 31 P nucleus)

'----------------+ 8

.---.~

(ij

c

C)

°Cii

o

:cn3 a:

_ p NMR spectrum of PF3 31

(One quartet peak; splitting by three 19 F nuclei)

i '-----------------+ 8 Fig. 12.2.19.1 19p_ and 31p-NMR spectra of PP3'

1850

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

(19 F_NMR)

(very low temperature)

iL.---

(Relatively higher temperature)

_

il...---

_

- - - - - . . Ho (b)

ra ry

Fig. 12.2.19.2 19p-NMR spectra of NP3. (a) Low temperature (ca. -205°C); (b) High temperature (ca. 20 - 30°C).

(I =±). H

the fifth axial P

er

e

t.m

e/

th e

al

ch

em

yl ib

It is suggested that at low temperature, the slow molecular motion (tumbling motion) is very much efficient for the quadrupole relaxation and the F-atoms see the time average zero nuclear spin state (i.e. 1= 0) at the N-nucleus (cf quadrupole relaxation is much faster than the NMR transition time). But at higher temperature, the rapid molecular motion is not so effective for the quadrupole relaxation and the life time of a particular nuclear state of the 14N nucleus is sufficiently high to split the 19F-NMR signal (cf Sec. 12.2.16). Note: In the fluxional molecules, at higher temperature, the different peaks merge. But for NF3 , the reverse situation arises because the reasons are different in the two cases. 8. WF6 ·L (complexes of WF6) and W20 2F9": The general formula of the complexes, tungsten hexafluoride is given by WF6 ·L (L = ligand). At the early date, it was assumed to have the 7 coordination number. The 19F-NMR spectrum of the complex, WF6 ·L shows the three peaks with the intensity ratio 4: 1: 1. It indicates the presence of three types of 19F as: 4F, IF, IF. This is not in conformity with the 7 coordinate structure of WF6 ·L. The NMR signals for 19F support the structure as: [WFsL]+P-. In octahedral [WFsL]+ (C4v )' 4F atoms are at the basal plane while the axial positions are occupied by the fifth F and L. The 4F atoms at the basal plane are equivalent (related by the C4 axis) and they produce the strongest peak which is split into a doublet peak by The NMR signal due to the axial P is split into a quintet (intensity ratio,

lic

k

1:4:6:4: 1, see Pascal's triangle) by the 4 basal 19P-centres (I =±). The remaining unsplit peak

C

is due to the' P- which resides outside the coordination sphere.

i~~~J-7i i/Wj~i

F

F

F-------- --------F F ([WLFs]+ F-)

The 19F-NMR spectra of W 20 2F g- shows: a doublet of intensity 8 and a nonet of intensity 1 (i.e. intensity ratio, 8:1). This observation is in conformity with the structure OWF4-F-WF40- having two

1851

SPECTROSCOPIC METHODS AND OTHER PHYSICAL METHODS IN CHEMISTRY

e/

th e

al

ch

em

yl ib

F

ra ry

types of F atoms (8: 1) where one F-site acts as a bridging site. The 19F-NMR signal of the bridging fluoride is split into 9 components by 8 equivalent basal fluorides. The 19F-NMR signal of the basal F-sites is split into a doublet by the bridging F-centre. 9. HF2-: The linear structure [F ... H ... F]- is in conformity with the 19F_NMR spectrum. Here both the fluorines are equivalent and they produce one 19F-NMR signal which is split into a doublet by the proton. 10. SF4 and O==SF4 : Both the species show two triplet peaks in 19F-NMR spectroscopy (32S, I =0). Their structures are related with the trigonal bipyramidal geometry (i.e. sp3dhybridisation of S). In each case, there are two types of F-atoms: 2 axial F and 2 equatorial F. Each of them will give two 19F-NMR signals and each signal will be split into a triplet by the other two Fatoms. Thus the trigonal bipyramidal structure supports the NMR spectra: two triplet peaks for 19F-NMR spectroscopy in both the compounds.

lic

k

H

er

e

t.m

The other possible symmetrical structures, i.e. planar SF4 (D4h ), square pyramidal OSF4 (C4V ) should give only one unsplit NMR signal for 19F because in such cases, 4F atoms are equivalent. 32S (I = 0) is a nonmagnetic nucleus' and it does not come into the picture to split the 19F_NMR signals. Note: Because of the rapid exchange between the nonequivalent F-atoms, for SF4 (cf. Sec. 12.2.15) only one sharp signal is obtained (at room temperature). At very low temperature (ca. -100 C C), two separate peaks are observed because at such low temperature, the exchange rate is sufficiently slow with respect to the NMR time scale. It happens so for also SeF4, TeF4'

C

11. [Rh(CN)sH]3- and other transition metal-hydrides like carbonyl hydrides (Sec. 12.2.6): The PMR signal (D =- 10.6 ppm) for [Rh(CN)sH]3- is a doublet one due to the splitting by Rh (I =

~)

It indicates the existence of the Rh-H bond. Here it is worth mentioning that in the metal hydride complexes (cf. Sec. 12.2.6), the protons are strongly shielded and their chemical shifts commonly lie in the range -5 to -60 ppnl relative to TMS (cf. 8 = - 7.5 ppm, -9 ppm, -11 ppm for [HMn(CO)s], [HFe(CO)4]- and [H 2Fe(CO)4] respectively). Thus in terms of chemical shift, such protons are typical (i. e. highly negative 8 value) and very few other protons appear in this range. Thus the M-H protons can be easily identified. The very high upfield resonance (8 = -5 to -60 ppm) for the hydride ligands bound to the transition metals may arise from the diamagnetic shielding by the metal d-electrons. In such cases, the transition metal centres generally lie in the low oxidation states and the electronegativities of such metal centres and hydrogen become comparable and in some cases, electronegativity of hydrogen may be even larger

1852

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

al

ch

em

yl ib

ra ry

than that of the low valent metal centre. Thus under the condition, X(H) ~ X(M) and the hydridic character of the transition metal hydrides, the phenomenon, diamagnetic shielding apparently sounds well. But this diamagnetic shielding cannot alone explain the too high negative values of chemical shift. Here it is interesting to note that in [HCO(CO)4] or [H2Fe(CO)4], the metal bound hydrogen is acidic in character but it shows the negative value of 8 (z - 10 ppm). Acidic proton but negative 8-value! This is unusual. ' All these observations strongly indicate that besides the diamagnetic shielding, some other factors also contribute to make the large negative value of 8 for the hydrogen bound to the transition metal centre. This factor has been argued as the paramagnetic shielding by the incompletely filled d-electron subshell of the transition, metal centre (Refs. A.D. Buckgham et aI, 1 Chem. Soc., 2747, 4583, 1964; L.A. Lalancette et aI, J. Am. Chem. Soc., 86, 5145, 1964). This paramagnetic screening can occur even when the metal centre is itself diamagnetic. This paramagnetic screening becomes quite important when there are accessible low-lying excited electronic states. It happens so in the transition metal complexes with the incomplete d-electron subshell. Effect of the paramagnetic metal centres on the PMR signals of the ligands has been discussed in Sec. 12.2.19(26). 12. [MO(CO)3H2(PPr~2]: It may lead to the two possibilities: MO(1l2-H 2) (dihydrogen complex) and cis-M(H)2, (i.e. di~ydrido complex). X-ray and neutron diffraction studies could not solve the problem without any ambiguity. However, the PMR studies on the complex, [MO(CO)3HD(PPr~2] solved the problem.

I

) [

e/

HD

MO(CO)3HD (pprj)2] (18e)

t.m

(16e)

th e

[ Mo(CO)iPPrj)2]~[ MO(COhH2(pprj)zJ (18e)

J H-D = 33.5 Hz

k

H

er

e

The PMR spectrum of [MO(CO)3HD(PPri)2] shows the H-D coupling constant (JH- O ) about 33.5 Hz which is comparable to the value IJH_O (43.2 Hz) found in the PMR spectrum of free H-D. H It may be noted that the coupling constant 2J H- D is only 1 - 2 Hz as in the system like M< . D

C

lic

Thus the high JH- D constant strongly suggests that in the complex, HD remains as an

undissociated species, i.e. IJ H_ O '

M~(l]2_~D), i.e. [

Mo I JB _ Hb ). Thus the IIB-NMR spectrum ofB 2H 6 gives a triplet of triplets (Fig.

t.m

(ct.

±+ 3)

e/

can be split into a triplet ( = 2 x 2 x

th e

al

• IIB-NMR spectrum: The two B-centres are equivalent and they give one signal which is split into a triplet by the two equivalent terminal protons (H t ). Each component of the triplet

e

12.2.19.5).

er

• PMR spectra (considering the coupling with liB): The equivalent terminal protons give a

%+ 1 = 4) by II B ( I = %)

k

H

PMR signal (at low field) and the signal is split into a quartet ( = 2 x

C

lic

nuclei. The bridging and terminal protons are weakly coupled and each component of the quartet is weakly split into a triplet ( = 2 x 2 x

±1) +

by the two bridging protons

(ct. IJ

H' _B

> 2 J H' -Hb ).

Thus the PMR signal of the terminal protons consists of quartet of triplets (Fig. 12.2.19.5). The PMR signal (at higher field) of the bridging protons is split into a septet ( = 2 x 2 x by the two equivalent lIB protons into a quintet

%+ 1 = 7 )

(I =%) nuclei. Each component of the septet is weakly split by 4 tenninal

e

J Hb -B > 2 J Hb -H' ). Thus, the PMR signal for the bridging protons is basically

a septet of quintets (Fig. 12.2.19.5). The above conclusions are made by considering the 1st order effect and ignoring the presence of lOB (which is about 19% in abundance and I = 3). However, in reality, complications arise from the 2nd order effect and the NMR active less abundant lOB nuclei.

1856

ra ry

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

Septet of quintets

(Stick diagram for H

yl ib

Triplet of triplets (Stick diagram for 11 8 )

b

)

ch

em

Fig. 12.2.19.5 Considering the 1st order effect, the stick diagrams (qualitative representation; not in scale) of the NMR spectra of liB, Hb and H t ofB 2H6 • Note: For this symmetrical molecule, both sides are identical. Thus the NMR spectra of liB and terminal hydrogen CH), of both sides are identical (cf the PMR spectra of ring protons of 0- and p- disubstituted benzene, Sec. 12.2.8F, xiii). For the PMR spectrum of the bridging hydrogen (Hb), the whole molecule is to be considered.

lic

k

H

er

e

t.m

e/

th e

al

18. B3"8- (cf. Sec. 12.2.15): The llB-NMR spectrum gives a single peak having 9 fine components, i. e. nonet. It indicates the 8 protons equivalent to split the 11 B-NMR signal. The rapid intramolecular rearrangement (i. e. tluxionaiity) makes the three B-centres equivalent and 8 protons equivalent.

C

In the proton decoupled IIB-NMR spectrum of B3H g-, only one unsplit signal appears for the three equivalent B-centres. The 8 equivalent protons give a single PMR signal which is split into 7 fine components by three equivalent lIB (I = 3/2) nuclei.

%) about 80%; lOB (I = 3) about 20%.

Note: The natural abundance of boron isotopes is: 11 B ( 1 =

The PMR signal split by the relatively less abundant lOB isotope is of relatively poor intensity and in many cases, the lOB-multiplet is not observable. The coupling constant for JOB is relatively smaller than that of JJB because of the larger gyromagnetic ratio of 1JB.

19. Structure of B10"14: The IIB-NMR spectrum (decoupled from the protons) shows 4 peaks with the intensity ratio 4:2:2:2. It indicates that there are 4 types of B-atoms in the structure. These are: 4 equivalent B nuclei; three different (i.e. nonequivalent) pairs of B-nuclei. It may be noted that there is no B-B coupling. The skeleton structure of B 10H l4 is in conformity with the IIB-NMR spectrum.

1857

SPECTROSCOPIC METHODS AND OTHER PHYSICAL METHODS IN CHEMISTRY

110----

_

(a)

Ge~ C3Ge, I = ~), BH:;, BFi (lOB, 1=3; liB, 1= %): Because of the tetrahedral (i.e. symmetric

yl ib

20.

ra ry

Fig. 12.2.19.6 (a) Structure of B 1oH14; 4 equivalent B-centres are denoted by *B; other 3 nonequivalent pairs (a, a; b, b; c, c) are evident in the structure; (b) Proton decoupled llB-NMR spectrum of B lOH 14 .

by 73Ge. For

B~-, the PMR signal consists of a strong quatret

al

~ + I)

th e

components (2 x I x

ch

em

arrangement of the ligands, i.e. H-, P-) around the quadrupole nucleus (i.e. I > ! ), the quadrupolar 2 relaxation is insignificant (Sec. 12.2.6) and the quadrupole nucleus can split the IH and 19p_NMR signals. All the hydrogens in GeH4 are equivalent and the PMR signal is split into 10 fine

t.m

e/

(splitting by the more abundant II B nucleus of I = ~ ) and a weak septet (splitting by the less 2 abundant lOB nucleus of I = 3) (cf abundance of lIB and lOB about 80% and 20% respectively). Similarly, the 19p-NMR signal Of BFi consists cf a strong quartet and a weak septet (see Pig.

= PdII, PtII): Here the 31P-centres are non-equivalent and two 3Ip_NMR

er

21. [M(PR3)(PR~)X2] (M

e

12.2.19.8).

C

» 2Jp-P(cis).

lic

k

H

signals are expected for both the cis- and trans-isomers. Each signal is split into a doublet by the other nonequivalent 31P centre. The magnitude of this splitting is determined by the coupling constants 2Jp_p which are different for the cis- and trans- isomers. The cis- and trans- isomers of these square planar complexes can be distinguished by considering the 2Jp _p values: 2JP-P(trans)

2Jp _ p

(trans)

i~--7r3 i~--7r~

.l/M~i

R3P- - - - - - - - - - - - - - - X

~/M~i

X- - - - - - - - - - - - - - - PR3

("ans~

(c~~

2J p _

p

(trans) »2J p _

p (cis)

The complex, [PdCI2(PMe3)2] is quite interesting in terms of its PMR spectrum.

1858

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

C:~--7PMe3

Me3~~- - - - - - - - -- / - - -- -91

: I

:6 ~:2JI/ I

H

Pd

I

:/ :

I

: r

i/Pd~

H

I

./C"

1/

CI- - - - - - - - - - - - - - - - .P

H

CI· - - - - - - - - - - - - - - - - - - - -PMe 3

/ "'Me Me

(cis-isomer) 2Jp _ p

H

« 2J p _

H

»

4Jp-H

ra ry

(trans-isomer) 2J p _ p »2J p _

2J

yl ib

The two P-centres are chemically equivalent but they are not magnetically equivalent because 2Jp_H and 4Jp_H are of different magnitudes, i.e. the two P-centres are not magnetically equivalent.

ch

em

• For the trans-isomer, 2Jp_p is much larger than 2Jp_H. Thus the PMR signal (for the Me-proton) is split into a triplet one (1 :2: 1) by two chemically equivalent 31P-centres (which behave as the equivalent nuclei to split the PMR signal). In such cases, the two chemically equivalent but magnetically nonequivalent 31P-centres remain virtually coupled to split the PMR signal.

th e

al

• For the cis-isomer, 2Jp_H is much larger than 2Jp_p and the PMR signal (Me-proton) is split into a doublet by the attached 31p centre (i.e. 2Jp_H). Here further splitting by the other nonequivalent 31P-centre is ignored because 4Jp_H is negligibly small.

t.m

e/

22. cis- and trans-isomers of [PtBrCI(PR3)2]: In the trans-isomers, the two 31p centres are equivalent (related through the C 2 axis and crv) but the two 31p centres are nonequivalent in the cisisomer. Thus the trans-isomer gives one 31p_NMR signal but the cis-isomer gives two 31p-NMR

signals (each may be split into a doublet by the other nonequivalent P-centre).

H

er

e

For the trans-isomer, the 31p-NMR signal may be split into a doublet through the IJp_pt coupling 1 (cf 1=2" for 195Pt). For the cis-isomer, the two 31p-NMR signals may be split by both 195pt and the other nonequivalent 31P-centre giving rise to a double quartet (cf

1

1= -

C

lic

k

for both 31p and 195Pt) 2 whose shape depends on the relative values of 1Jp- Pt and 2Jp_p. The coupling constants 1 J 195 31 p (cis) 1 J 195 31 (trans) can also distinguish the isomers (see the next example). Pt Pt- p

22. J Pt-p for the cis- and trans- isomers of [Pt(PR3)2X2]: In each case, the 31P-centres are equivalent producing one 31p-NMR signal. The nuclear spins are:

1

"2

for 195pt and

1

"2

for 31p. In general, for

such square planar complexes, Jpt_p(cis) JPt_p(trans), e.g. J(cis) =3.62 Hz and J(trans) =2.46 Hz for [PtCI 2(PR 3)2] (R = n-Bu). This difference in the coupling constants can be understood by considering the dJt(P)-dJt(Pt) interactions. If the complex is supposed to remain in the xy-plane and in the trans-isomer, two trans- P atoms are along the x-axis, then the d-orbitals of two P-atoms can overlap with the dxz and dxy orbitals (i.e. two d-orbitals) of Pt. On the other hand, for the cisisomer, the two cis- P atoms (along the x- and y- axes) can overlap with the dxy, d yz and dzx orbitals (i.e. three d-orbitals of Pt). This greater x-bonding interaction between the P-d-orbitals and Pt-d-orbitals in the cis-isomer can explain the sequence of the coupling constants, J(cis) ) J(trans).

1859

SPECTROSCOPIC METHODS AND OTHER PHYSICAL METHODS IN CHEMISTRY

ra ry

Fig. 12.2.19.7 Schematic representation of one n-type interaction between the two trans-PR 3 ligands via the dxy orbital of Pt(II).

For both the isomers, the single 31p_NMR signal is split into a doublet through the 1 J 195 Pt_31p

yl ib

coupling but the difference in coupling constants 1 J Pt-P for the two isomers can distinguish

the isomers.

H

er

e

t.m

e/

th e

al

ch

em

It has been noted that th~ coupling constant 1 J Pt-P is very much sensitive towards the nature of the trans-ligand (with respect to the PR3 ligand). The coupling constant value depends on the trans-influence property of the ligand (trans to PR3). IJPt_p (for the trans-ligands): CI- ~ Br- ) NH 3 ) R- ) PR3 Trans-influence: PR3 ) R- ) Br- ) CI- ) NH 3 Bond weakening by a trans-ligand at the trans-position where PR3 resides lowers the Pt-P coupling constant. It is expected because the coupling between Pt and P is attained through the Pt-P bonding electron. 23. RoPF 5-0: In this trigonal bipyramidal (TBP) geometry, the more electronegative F-atoms preferably occupy the axial positions (cf. in Me2PF3, Me groups in the equatorial plane while in (CF3)2PF3, CF3 groups in the axial directions; Bent's rule). If the molecule is stereochemically rigid, then the axial and equatorial F-atoms appear as the nonequivalent centres and these can be identified by 19F-NMR spectra. The coupling constants (Jp- F) for the equatorial F-centres are relatively

k

larger, i.e. 1 J 31p_19 F(eq) » 1 J 31p_19 F{ax)' If the TBP geometry is considered to have two different

C

lic

sets of hybrid orbitals (i.e. sp 3d == Sp2 + pd) then the equatorial bonds are formed by using the Sp2 hybrid orbitals (s + px + Py' if the trigonal plane lies in the xy-plane) of P while the axial.bonds are formed by the pd-hybrid orbitals (Pz + dz2, axial groups along the z-axis) of P. The equatorial P-F bonds can use the s-orbital (which ha~ a finite probability of the existence of electron to the closest distance towards the nucleus) of P and this gives a better coupling interaction between the nuclear spins of 31p and 19F (cf. explains,

1JP-F(eq) ) 1Jp-F(ax)

I JC ( )

sp -

H

> IJ C (2) > IJ C (3) for I3 C, Sec. 12.2.9). It sp -H sp -H

and the difference in these two coupling constants is about 175 Hz.

Note: Here it may be mentioned that the 5 coordinate trigonal bipyramidal molecules are very often fluxional in nature (see Vol. 2). For example, in PF5 though there are two types of F-atoms (3 equatorial and 2 axial) but instead of two 19F-NMR signals, only one doublet signal is obtained. In fact, Berry's pseudorotation leads to the rapid interchange between the axial and equatorial F-atoms and the NMR method 'sees' only the time averaged F atoms. In other words, the NMR method cannot distinguish between the axial and equatorial F-atoms and all the five F-atoms appear equivalent. This is why one 19F-NMR signal is obtained and this is split into a doublet by

1860

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

31

p( ±). Simil~ly, 1=

other trigonal bipyramidal molecules like [Sb(CH3)s]

(one~MR signal),

[Fe(CO)s] (one 13C-NMR signal) also indicate that all the 5 ligands appear identicaf in the NMR time scale. It may , noted that the NMR studies at very low temperature may distinguish the axial and equatorial ligands because at the very low temperature, the rate of intramolecular exchange between t~e axial and equatorial ligands is slowed down. In PCI2F 3, the 19F-NJy1R spectral studies can detect the equatorial and axial F-atoms at very low temperature (ca. -145°C, Fig. 12.2.15.4). 24. Structures of [M3(C9)12]: For M = Ru, Os, (i.e. heavier congeners) the nonbridged structure is favoured while for M =Fe, the bridged structure is favoured (see Ch. 9 for explanation). These two possible structures can be distinguished by the 13C_NMR spectroscopy. L

ra ry

j

yl ib

L",I/ L

L"" I

em

axL/F~

c

ch

eq

c~

al

-----Fe---------- --- -~-) eqL/I~ c=o

e/

th e

~L

M = Ru, Os

= axial CO,

e

axL

t.m

L=CO,

~f Fe

/1'"L L

(C 2V)

(L=CO)

L

[Fe 3 (CO)12]

er

eqL = equatorial CO

C

lic

k

H

For the nonbridged structure (D3h symmetry), there are two types of CO groups (i.e. axial and equatorial) and two 13C-NMR peaks are expected. If the structure of [Fe3(CO)12] is supposed to be stereochemically rigid, then the symmetry operation on the C 2-axis passing through the Fe-centre (bearing only the terminal groups) and between the two bridging CO groups interchanges the pairs of CO groups, i.e. 6 nonequivalent CO pairs. Thus it should give six 13C-NMR signals. If the bridged structure of [Fe3(CO)12] rapidly interconverts into the nonbridged structure (i.e. fluxional character) then the axial terminal CO groups (of the nonbridged structure) and bridging CO groups (of the bridged structure) are rapidly interchanged (faster than the NMR time scale). In this interconversion process, the equatorial CO groups remain unchanged. Thus two 13C-NMR signals (i.e. one for the equatorial CO groups and one for the axial and bridging CO groups which cannot be distinguished by the NMR method because of their rapid interconversion) are recorded. In fact, at very low temperature, the bridged structure predominates (and the interconversion rate is slow), then the complex 13C_NMR spectra (consisting of 6 peaks) appear but at higher temperature, the fluxionality (i.e. rapid interconversion between the axial and bridging CO groups) is attained and two 13C-NMR peaks are noted. 25. Structures of [Fe(COs)], [Sb(CH3)s], PFs : In terms of its trigonal bipyramidal structure of D 3h symmetry, there are two types of CO groups (i.e. 3 equatorial CO groups and 2 axial CO groups).

1861

SPECTROSCOPIC METHODS AND OTHER PHYSICAL METHODS IN CHEMISTRY

Infrared and Raman spectra of [Fe(CO)s] indicate the presence of these two types of CO groups (cf. Fig.·;12.1.11.1). The rapid interconversion between the axial and equatorial CO groups (i.e. fluxionC;ll character) leads to one 13C-NMR signal (cf ir-spectrum Fig. 12.1.11.1). The infrared and Raman techniques are the much faster techniques while the NMR technique is a much slower process (compared to the interconversion rate of the axial and equatorial·CO groups) and it actually records the time average structure of [Fe(CO)s]. Because of the same ground, the trigonal bipyramidal molecule [Sb(CH3)s] shows only one PMR signal. A similar situation arises for PFs where only one 19F-NMR signal (split into a doublet by 31p) is obtained.

em

centre is paramagnetic (as in the present tetrahedral complex of NiH), the chemical shifts are enormously affected.

ch

NiBr2

yl ib

ra ry

26. Structure of [NiBr2{PMe(p-MeOC6H4)2}2] (cf L. H. Pignolet et aI, J. Am. Chem. Soc., 92, 1855, 1970; E. A. Lalancette et-al, ibid, 86, 5145, 1964): It can have two isomeric structures of [Ni(PR3)2X2] discussed in Chapter 2). These are: paramagnetic tetrahedral and diamagnetic square planar. The chemical shifts (8 in ppm) of the PMR signal of the protons present in the phosphine ligands are highly sensitive to the magnetic character of the metal centre to which the phosphine ligand is attached. If the metal

e

t.m

e/

th e

al

In the square planar complex, the diamagnetic character of Ni(ll) cannot affect the PMR signals. For this square planar complex, three PMR signals appear for the methyl (CH 3), methoxy (OCH3)..a nd4 aromatic (though they are not equivalent, they are not resolved) protons. In the tetrahedral complexes, the paramagnetism of the metal centre can differentiate the orthoand meta- protons (with respect to the P atom) because of their different degrees of interaction with the unpaired electrons of the metal centre (cf p- and o-protons more upfield while m-

H

er

protons relatively downfield). The resonance position of the P-CH3 protons is also greatly affected by the paramagnetism of the metal centre and it lies beyond range. The positions of

lic

k

different protons in the square planar and tetrahedral complexes are:

C

Td comPlex}

Ho :

meta-H -27.5

OCH 3 -7.0

TMS 0

ortho-H +8.2 (upfield)

CH3 protons lie } beyond the range.

(Relative values of H o for IH resonance)

Square planar } complex

increasing magnetic field Ph-H (aromatic protons) -7.6 (no resolution for the o-and m-protons)

OCH 3 -3.85

TMS -1.95

o

(Relative values of H o for 1H resonance)

increasing magnetic field At very low temperature both tetrahedral and square planar geometries are frozen out into an equilibrium mixture and the PMR signals of the both the geometries appear. At a relatively higher temperature, there is a rapid interconversion between the two geometries (i.e. stereochemical

1862

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

nonrigidity) and the PMR spectrum reveals only the time averaged environment for the each type of protons as follows: meta-H

OCH3

ortho-H

TMS

-12.2

-4.75

-3.2

o

--------+)

Increasing field

Ho

• Effect of the unpaired electrons of the paramagnetic metal centre on the PMR signals of the ligand protons: Paramagnetic Ni(ll) (tfS) (in the tetrahedral complex, e4 ti) possesses both the filled and half-filled dn-orbitals which can participate in the dn(Ni) ~ dn(P) interaction (cf

th e

al

ch

em

yl ib

ra ry

phosphorous bears the vacant d-orbital). In the said drc-drc interaction, if the filled Ni(ll) d-orbital and vacant P d-orbital are involved, then it will simply establish a n-bond between Ni(ll) and P. But, in the said interaction, if Ni(II) uses a halffilled d-orbital, then the unpaired electron ofNi(II) will be partially transferred on P and this transferred electron can again be partially delocalised into the n-system of the phenyl ring through the dn(P)-Pn(C) interaction.

t.m

e/

Thus accumulation of higher negative spin density at the ortho- and para- positions causes the upfield shifts for the ortho- and para- protons. Because of the low negative spin density at the meta-position, the meta-protons experience the downfield shifts. Here it is worth mentioning that

e

this spin transfer (i.e. unpaired electron transfer) from the metal centre to the ligand can also occurs through the a-system but this effect rapidly decreases with the distance from the metal

er

centre.

lic

k

H

Note: Paramagnetic shielding on the metal bound hydrogen (i.e. M-H) can occur even for the diamagnetic metal centre having the partly filled d-shell (see example 11 of this section). It explains the highly negative value of 8 for the hydrogen bound to the transition metal centre.

C

27. [(Tl 5-Cp)Rh(Tl 2-C2H4)2]: PMR spectra (cf Fig. lO.11.3.1(b» of the ethylenic protons at different temperatures indicate the rotation of the coordinated ethylene around the metal-olefin bond. At very low temperature when the activation energy for the said rotation is not attained (i. e. rotation is kinetically prevented), two different types of ethylenic protons are indicated by two PMR signals but only one PMR signal for the time averaged environment of the proton appears at the relatively higher temperature when the said rotation is kinetically allowed.

28. NMR spectral feature of the low-spin Co(III) complexes: The NMR studies of different low . spin Co(lll) complexes (cf 1='2 for 59CO) indicate that the chemical shift (8), i.e. resonance 2

frequency depends on the energy difference (~E) between the diamagnetic ground state eAIg) and the paramagnetic excited state T 1g ). The chemical shift occurs due to contribution from this paramagnetic excited term. This is why, contribution of this excited state to the chemical shift increases with the decrease of the energy difference (~) between the IA lg and ITlg spectroscopic states and this energy difference can be easily obt~ined from their electronic spectra.

e

1863

SPECTROSCOPIC METHODS AND OTHER PHYSICAL METHODS IN CHEMISTRY

In fact, the external magnetic field allows the mixing of the wavefunctions of the ground state eAIg) and excited state eTIg). The induced current generates a magnetic field and deshields the nucleus (i.e. local paramagnetic deshielding measured by op which is inversely proportional to Llli, i.e. op oc 1/dE). Thus with decrease of dE, 8 (59 CO) increases. With the decrease of Llli, contributon of op (-ve) increases, i.e. Hex (experienced magnetic field by the nucleus) increases to make the resonance frequence larger (cf. vres oc Hex).

It has been noted that the resonance frequency increases with the decrease of the energy difference (.1£) for the IA lg ---) ITlg spectroscopic transition. K 3[Co(CN)6]

[Co(en)3]CI3

[Co(NH3)6]CI3

[Co(acac)3]

32,100 4.417

21,400 4.449

21,000 4.453

16,900 4.737

ra ry

Complex: Llli (em-I): v res (MHz): 8 (59CO):

Increasing trend

t.m

e/

th e

al

ch

em

yl ib

29. Distinction of cis- and trans- isomers of [Ti(acac)2(OEt)2]:

er

e

CH 3 (cis)

H

(trans)

C

lic

k

In the trans-isomer, the CH2 protons of the ethoxide group are equivalent in terms of the mirror plane (0) and they give a single PMR signal. In the cis-isomer lacking in the mirror plane, the CH 2 groups (of DEt) are not equivalent and they give two separate PMR signals. The CH3 protons of the DEt groups also give the separate PMR signals for the cis-isomer. 30. Distinction between the fac- and mer- isomers of [Pt(CH3)3(NH3)3] (cf. Example 16 of [RhCI3(PPh3)3]: In the fac- isomer of [Pt(CH3)3(NH3)3] all the Me-groups are in cis-configuration and they are equivalent (related through the C 3 axis). These methyl protons give one PMR signal which is split by 195pt

(I = ~ )(cf.

2 J I H_ 195pt ""

75Hz). In the mer- isomer, there are two types

of Me-groups (1:2) and two PMR signals (intensity ratio 1:2) for the methyl protons are found. Df course, these two signals are also split by I95pt. 31. BF4-, BH4- (isotopic shift for lIB and lOB): Boron is present in a symmetrical tetrahedral environment of the ligands (i. e. H-, P-) and consequently, the electric field gradient (q) is zero. This is why, the quadrupolar relaxation of

11 B

(1=

I9p-NMR signal or IH-NMR signal is split by boron.

~) or lOB (I = 3) is not significant and the

1864

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

The 19F-NMR spectrum shows a well-resolved quartet (strong signal) and a septet multiplet (weak signal) (Fig. 12.2.19.8). The quartet ( = 2 x

%+ 1) signal i~ due to the predominant iso-

topic species 11BF4- (ca. 80%, I = ~ ) while the weak septet (= 2 x 3 + 1) signal is due to the less 2

Quartet for BF4-

al

ch

em

yl ib

ra ry

abundant isotopic species lOBF4- (ca. 20%, 1=3). The separate signals of lOBF4" and lIBF4" show the isotopic shift of about 0.048 ppm. The isotopic shift occurs because the vibrational states of the two isotopic species slightly differ and consequently the electron distribution patterns in loB_F and lIB-F bonds are also different. The coupling constants are: 1 J 19p_llB = 1.2 Hz, I J 19 F _lO B = 0.4 Hz. The bigher coupling constant for lIB is due to its higher gyromagnetic ratio (cf. YN(lIB): YNeoB) ::= 3.0). Similarly, for BH4-, a strong quartet PMR signal due to the more abundant species 11BH4appears and a weak septet PMR signal occurs due to the less abundant 10BH4- species. Here these two multiplets overlap (cf. chemical shift values for 19F cover a much wider range than those for 1H; Sec. 12.2.5B).

e

t.m

e/

th e

Isotopic shift

er

Septet 10

-

lic

k

H

for BF41--~

C

----+ Ho

Fig. 12.2.19.8 19F-NMR spectrum (qualitative representation) of BFi ion in aqueous solution.

32. Structure of Al(B~)3: The

27Al ( I

=

%) decoupled PMR spectrum shows a 1:1:1:1 quartet

(Fig. 12.2.19.9) due to splitting by llB (80%, I = ~ ) (ignoring the splitting by the less abundant 2

isotope lOB). The lIB-NMR spectrum (decoupled from 27 AI) shows a 1:4:6:~:1 quintet (Fig. 12.2.19.9) due to coupling by 4 equivalent protons around the B-centre. It indicates the rapid exchange between the terminal and bridging H-atoms. The lIB-decoupled PMR spectrum shows a broad signal composed of six broad lines of equal intensity (cf. splitting of the PMR signal by I = ~ of 27 AI). It indicates that all the 12 hydrogen atoms are equally coupled to AI. 2

Thus the AI-centre retains three rapidly tumbling BH4 units.

1865

SPECTROSCOPIC METHODS AND OTHER PHYSICAL METHODS IN CHEMISTRY

(a) 27AI-decoupled PMR spectrum

(b)

27 AI

decoupled

11 B _NMR

spectrum

(c) 11B-decoupled PMR spectrum

Fig. 12.2.19.9 NMR spectra (qualitative representation) of AI(BH4)3o

The exchange of terminal and bridging H-atoms probably occurs through the singly or triply bridged

intermediates (Fig. 12.2.19.10).

~

H

ra ry

/ H, , /

(H 4 B)2 AI" / ' B "

H

ch

em

yl ib

H

th e

al

Fig. 12.2.19.10 Probable mechanisms for the exchange of bridging and terminal H-atoms in AI(BH4)3o

H

er

e

t.m

e/

33. Fischer carbene (OC)sCr=C(Ph)(OMe): The resonance in this carbene introduces a partial double bonded character in the C-O linkage (cf. observed bond length 133 pm is shorter than the typical C-o single bond of about 143 pm but significantly lon~r than the typical C 0 bond of about 116 pm). But this double bonded character can sufficiently slow down the rotation around the C-O bond at low temperature to cause the cis-trans orientation of the Me-group. In fact, at low temperature, two PMR signals for the Me-protons "are recorded. But at higher temperature, the said rotation becomes faster and the cis-trans interconversion rate becomes faster than the NMR time scale, i.e. NMR records only an average signal producing a single PMR peak

C

lic

k

for the Me-groups.

/ (OC)sCr=-=-=-=c,

Ph

"'.

O -CH 3

(cis-form)

Ph

(oc)cr==c( o

5

"'.

,

HC/' 3

(trans-form)

12.2.20 Application of NMR Spectroscopy in Identification of Rotational Isomerism (cf.Sec.12.2.10, Figs. 12.2.10.1-2) (i) BrCI2C-CBrF2: Let us consider, BrCI2C-CBrF2 which can adopt three different conformations in which the 19p centres are not equivalent (cf Pig. 12.2.20.1) but if the free rotation around

1866

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

the C-C bond can occur rapidly (at a relatively higher temperature), then the F-atoms experience a time averaged environment and one 19F-NMR signal appears. At a lower temperature, the available thermal energy is less than the internal rotational energy barrier and then the rotational isomers can be frozen and two different 19F-NMR signals appear. These signals are mutually split and a complex spectrum is observed. Br

CI

F

CI

F

F

F

~ ~

~ ~

Br

CI

Br

(I)

(II)

Br

(III)

yl ib

Br

ra ry

CI

em

Fig. 12.2.20.1 Rotational isomers of BrCI 2C-CBrF2 _ In (i) and (iii) two Br atoms are in gauche; and these two conformations are enantiomers and they have the same NMR spectra in an achiral solvent.

bH

cH

t.m

e/

th e

al

ch

(ii) BrCH2-CH2Br: In terms of the PMR signal, the rotational isomers (see Fig. 12.2.10.2) of BrCH2-CH2Br (where the four protons are chemically equivalent) can be detected at very low temperature when the rotation around the C-C bond is locked. (iii) CH2CI-CHCICH3 : Let us illustrate the detection of the rotational isomers of 1,2-dichloropropane, i.e. CH2CI-CHCICH3 in terms of the PMR spectra. There are 4 types ofH-atoms and one C-centre can act as an stereocentre. The aH and bH atoms are chemically equivalent but they are magnetically nonequivalent (see Sec. 12.2.10) when the free rotation around the C-C centre is locked. These two hydrogens (i.e. aH and bH) are the diastereotopic hydrogens.

er

e

I I. I I

H

aH-C-C-CH

k

CI

.A

P

d

'" *

,,'

C-C~

3

R

C

lic

(* stereocentre)

Diastereotopic groups

"'x·

0"'1

CI

J

A

In 1,2 dichloropropane, the 4 PMR signals appear (at very low temperature) for the four types of hydrogens (cf. Fig. 12.2.20.2). CH 3 b

aH -

H

I I

c

H

I I

C - C - C H3 CI

CI

CI CI

(Contd... )

1867

SPECTROSCOPIC METHODS AND OTHER PHYSICAL METHODS IN CHEMISTRY

(Contd... ) NMR signal of b H

NMR signal of a H

3 J(CH -H c) 3 2J

2Jab

~ 6 Hz

~ 11 Hz

3JbC~ 6 Hz 3Jac~ 8 Hz

!

Jbc

I

I

yl ib

ra ry

JUl Two doublets (i. e. quartet)

em

Two doublets (i.e. quartet)

ba

=

ch

Fig. 12.2.20.2 Qualitative representation of the origin of the PMR signals of the two diastereotopic protons (in terms of splitting tree) in 1, 2-dichloropropane.

t.m

e/

th e

al

a H: Its signal is split by b H and cH. It gives two doublets Diastereo - i.e. double doublets or a quartet. topic b H: It signal is split by aH and cH. It gives two doublets i.e. double doublets or a quartet. cH: It couples with aH, bH and three Me-protons. Me-protons split the signal into a quartet and each of the components of this quartet is split separately by aH and bH. Thus it gives a complex multiplet

H

er

e

signal. dH (i.e. Me-proton): Its signal is split into a doublet by CR. NMR Signal ofH

k Br

C

cF

lic

H

c F

CI

I

I

Br-C--C-H

I

b

CI

F

I

}

a F

a F b

F

Splitting by one diastereotopic b F } S.plittin g by ~n~ther dlastereotoplc F

(Two doublets of doublets)

Fig. 12.2.20.3 Qualitative representation of the origin of the PMR signals of I-bromo-2-chloro-l, 1, 2-trifluoroethane (splitting caused by three nonequivalent F-atoms, aF, bF and cF; bF and CF are diastereotopic in nature).

(iv) F2BrC-CHCIF: Let us illustrate the PMR spectrum of I-bromo-2chloro-l, 1, 2-trifluoro ethane, F2BrC-CHCIF where the two F-atoms attached to a C-centre are diastereotopic in nature. If the rotation around the C-C bond is locked, the rotational isomers can be detected and the

1868

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

b

yl ib

ra ry

magnetic nonequivalence of the diastereotopic P-atoms can be identified by 19p-NMR spectroscopy. The characteristic features of the PMR spectrum of this compound (cf. Fig. 12.2.20.3) are as follows: The PMR signal is split into a doublet by the P-atom attached to the same C-atom; then each of the component of this dou~let is split by the diastereotopic P-atoms separately; thus the resultant PMR signal shows two doublets of doublets (i.e. simply two quartets). The 19p-NMR spectrum of this compound is a complicated one. Three nonequivalent P-atoms are expected to give three NMR signals and each signal is split by the two other nonequivalent P-atoms and one H-atom. (v) FCI2C-CCI2F: 1, 2-difluorotetrachloroethane can have three possible conformations. Two of these conformations are magnetically equivalent. At room temperature, when the rotation about the C-C bond is very fast, only one 19p-NMR signal is recorded. However at a very low temperature (about -120°C), two 19p_NMR signals are obtained. F

em

CI

ch

CI

al

CI CI ~ ~

CI

b

F

~ ~

CI

b

F

CI

CI CI

(ii)

(iii)

k

(i)

H

er

e

t.m

e/

CI

th e

CI

CI

C

lic

Fig. 12.2.20.4 (a) Three possible conformations of FCI 2C-CCI 2F; (ii) and (iii) are magnetically equivalent. At room temperature, one 19F NMR signal (rapid rotation about the C-C bond; time average environment of F). At low temperature, two different 19F_NMR signals.

J

3 F,F

(anti)

F CI$b CI~ CI$CICI --::.CI$CICI ~

--;-

CI

CI

CI

SF' (i)

b

SF

b

F

-.-.-J~

3

J F,F

3JF,F

(anti) *-

3JF,F

SF

(gauche) (iii)

(ii)

i

CI

F

(gauche)

Fig. 12.2.20.4 (b) F-F coupling constant elF, F) differing in different rotational isomers (cf. Karplus Rule, Sec. 12.2.9).

1869

SPECTROSCOPIC METHODS AND OTHER PHYSICAL METHODS IN CHEMISTRY

12.2.21 Application of NMR Spectroscopy in Detection of H-bonding and in Distinction of Intramolecular and Intermolecular H-bonding These aspects have been discussed in Sec. 12.2.5.

12.2.22 Application of NMR Spectroscopy to Determine the Relative Amounts of the Keto-Enol Tautomers in a Tautomeric Mixture

yl ib

ra ry

Detection of the keto-enol tautomerism by using the PMR spectroscopy has been discussed in Sec. 12.2.5. For the tautomerism in ~-diketones or ~-ketoesters, to determine the keto-enol ratio, the relative intensities of the PMR signals of the )CH2 group (keto form) and the = CH- group (enol form) are to be determined from their PMR spectra. For acetylacetone, the intensity (in terms of area) ratio of the methyl and methylene protons in the pure keto form is 6:2, i.e. 3: 1 while the intensity ratio for the methyl and methine protons in the pure enol form is 6: 1. By measuring the peak areas of the methylene and methine protons for a tautomeric mixture of acetylacetone, it has been found as: keto form 20% and enol form 80%.

em

12.2.23 Application of NMR Spectroscopy for Determination of Exchange Rate Constant

al

ch

This aspects have been discussed in Sec. 12.2.15. The solvent exchange rate constant can be determined by considering the NMR peak broadening phenomenon due to the exchange process (cf Chapter 5).

th e

12.2.24 Application of NMR Spectroscopy and MRI (Magnetic Resonance Imaging) This aspect has been discussed in the author's book, Bioinorganic Chemistry.

t.m

e/

12.2.25 Fluxionality through the Intramolecular Rearrangement and NMR Studies

C

lic

k

H

er

e

These aspects have been discussed in detail in Sec. 10.9 (Vol. 2) and Sec. 10.11. Some examples are mentioned. 1. Fluxionality of PFsand PCI2F3in terms of 19F-NMR spectroscopy: These are discussed in Sec. 10.9 (Vol. 2) and Sec. 10.11. The 19F-NMR sectra of PCl2F 3 at different temperatures are given in Fig. 12.2.15.4. 2. Fluxionality and rotational isomerism by PMR and 19F-NMR spectroscopy: These are discussed in Sec. 12.2.20. in terms of PMR and I1B-NMR spectroscopy: This has been discussed 3. Fluxionality of B3 in Sec. 12.2.19. 4. Stereochemical nonrigidity of [NiX2(PR3)2] (polytopal rearrangement, tetrahedral ~ square planar) by PMR spectroscopy: This has been discussed in Sec. 12.2.19. 5. Fluxionality of [M3(CO)12] and [Fe3(CO)12] in terms of 13C-NMR spectroscopy: This aspect has been discussed in Sec. 12.2.19. 6. Fluxionality of Tl 3-allyl organometallic complexes by PMR studies: This aspect has been discussed in Sec. 10.9.6 (Vol. 2) and Sec. 10.11.3. 7. Fluxional behaviour of [Ti(Tl s-Cp)2(Tl 1-Cp)2] in terms of PMR studies: This aspect has been discussed in Sec. 10.9.6 (Vol. 2) and Sec. 10.11.3. 8. Fluxional behaviour of [(Tls-Cp)2Fe2(CO)4] and [Mo2(CO)6(lls-Cp)2] in terms of PMR and 13C-NMR spectroscopy: This aspect has been discussed in Sec. 10.9.6 (Vol. 2) and in Sec. 10.11.3. See Fig. 12.1.17.1 for the Mo-complex for the veo values. 9. Fluxionality of [Fe(CO)s)] in terms of the 13C-NMR spectroscopy: This aspect has been discussed in Sec. 10.11.1.

"8-

1870

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

10. Fluxionality of [RU(CO)3(Tl 4-CsH s)] in terms of the PMR spectroscopy: This aspect has been discussed in Sec. 10.11.3. 11. Fluxionality of [(Me2C=C=CMe2)Fe(CO)4] in terms of the PMR spectroscopy: This aspect has been discussed in Sec. 10.11.3. 12. Fluxionality of [Ta2(OCH3)10]: There are two bridging MeO groups. (ax)

"""I

/Ta (eq)

HaCO

Ta

1""'- / (8H

I

"'OCH a (eq) OCH a

OCH a""'(ax)

OCHa /OCH a (eq)

(ax)

a

ra ry

1./

CH 3

OCHa./0""" (eq) HaCO""

yl ib

(ax)

Bridging group

C

lic

k

H

er

e

t.m

e/

th e

al

ch

em

There are 3 types of OCH3 groups: 2 bridging; 4 axial and 4 equatorial. In terms of the symmetry operation, the 2 bridging groups are equivalent. Similarly, the 4 axial groups are equivalent and the 4 equatorial groups are also equivalent. If the structure is stereochemically rigid, there will be 3 types of PMR signals having the relative intensities 6: 12: 12, i.e. 1:2:2. It happens so at very low temperature. But at high temperature, all these three types of OCH3 groups mutually interchange rapidly (with respect to the NMR time scale) through the rupture of the bridging bond and only one PMR signal is obtained. 13. Scrambling of Me-groups in A12Me6: In this dimeric form, there are two types of Me-groups: bridging and terminal. At low temperature, two PMR signals indicate the two types of Me-groups but at high temperature, the rapid scrambling (through the formation of monomers followed by their recombination, i.e. Al2Me6 ~ 2AIMe3 ~ A12Me6) of the Me-groups is indicated by one PMR signal. This aspect has been discussed in Sec. 10.10. 14. Rotation of the coordinated olefin in [(Tl 5-Cp)Rh(C2H 4)2] (cf Fig. 10.11.3.16): Two different PMR signals for C 2H 4 at low temperature suggest two different types of ethylenic protons but the rapid rotation of the coordinated ethylenes around the Rh-C 2H4 bond at a higher temperature makes all the ethylenic protoris equivalent and one PMR signal is noticed. This has been discussed in Sec. 10.11.3. 15. Fluxionality of [(Tl 5-Cp)Fe(CO)2(Tl 1-Cp)] (cf. Fig. 10.11.3.3b): In this tetrahedral molecule, the ring whizzing of the a-bonded Cp (i.e. 1l1_Cp) can make the three different types of protons equivalent. It is proved by the PMR studies at different temperatures (cf. Sec. 10.11.3).

12.2.2.6 Two-Dimensional NMR -

ROSY, COSY, NOESY

Two dimensional NMR techniques like H/H, HlC, CtC COSY (2D-correlated spectroscopy); H/H, HlC, ROSY (2D-J-resolved spectroscopy); NOESY (2D-nuclear overhauser exchange spectroscopy) etc. have been found quite useful for structure determination specially in organic chemistry. The principles and application of these advanced NMR techniques are beyond the scope of this present book.

12.3 ELECTRON SPIN RESONANCE (ESR) SPECTROSCOPY A molecule or ion bearing the unpaired electron(s) absorb electromagnetic -radiation in the microwave region under the influence of an external magnetic field. This electromagnetic radiation causes the

1871

SPECTROSCOPIC METHODS AND OTHER PHYSICAL METHODS IN CHEMISTRY

transition between the electron spin energy states. This phenomenon is described as the electron spin resonance (ESR) or electron paramagnetic resonance (EPR) or electron magnetic resonance (EMR cf PMR). It occurs for the paramagnetic substances, e.g. radicals, molecules (e.g. NO, 02, N0 2, etc.) and transition metal complexes bearing the unpaired electron (s) e.g. Cu(II)-complexes.

12.3.1 Interaction between the Electron Spin and Magnetic Field: Some Basic Elements of ESR Spectroscopy

=~s(s + 1)

Ch1t}= S(zh1t )

yl ib

Ps

ra ry

The principle of ESR is very much similar to that of PMR. In PMR, the external magnetic field (Ho) interacts with a positively charged proton while in ESR, the magnetic field interacts with a negatively charged electron. For an electron of spin quantum number, s = 1/2, the magnitude of spin angular momentum (Ps) is given by:

i.e.

~s(s+l) (z:)c0se

(:1t) where e is the orientation angle of the spin angular momentum. It leads to:

= ~s(s + 1) cosS. From the knowledge of quantum restriction, m s can have the two values ±1/2 and

al

ms

ms(z:)

ch

= ms

em

The z-component of the spin angular momentum is given by

er

e

t.m

e/

th e

m s is called the magnetic spin quantum number. The m s values (±1/2) indicate the two possible orientations of the electron spin. In absence of any external magnetic field, these two orientations are degenerate. But in presence of an external field, the spin degeneracy is removed. The low energy state is characterised by the quantum number m s = -1/2 in which the spin magnetic moment (J..ls) is oriented in the direction of the field. On the other hand, the higher energy state corresponds to m s =+ 1/2 where the spin magnetic moment opposes the field. The magnetic moment due to the spinning motion of an electron is given by (see Sec. 1.10.17, Vol. 1):

k

21t

2me c

(the negative sign indicates that the direction of is opposite to that of Ps)

lic

2m e c

H

~s = -~ Ps = -~ x ~s(s + I) x!!.-;

C

=-2~s(s+1) (~J 41tm c =

e

-2~s(s + 1) JiB'

In general, J..l s =- g ~ s(s + 1)

J..l~;

g (called

Lande splitting factor)

:::=

2 (for a free electron)

A eh (4.8 X 10-10 esu) (6.62 x10-27 erg s) Jl B (ca11e d B 0 hr magneton ) = tJ =--- =---------.;.------~ 41tm e c 4 x 3.14 x 9.1 x 10-28 g X 3 X 10 10 cm S-1

= 9.27 X In SI unit,

10-21 erg

a-I

~B = f3 = ~ =9.Z74 X 10-24 JT- 1 41tm e

~s

1872

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

Energy of an electron in a magnetic field (Ho) is given by:

E=-ll z Ho =-(llscose)Ho =-(-gIlBJs(s+l)cose)Ho = gJIBmsHO 1 For the ±- values of m s, the energy values are:

= g~msHo' (denoting JIB

by~)

2

IJ 1

(

1

E 1 forms =-- =--gJIBHO =--g~Ho; E 2 2 2 2

(forms =+-2IJ =-gJIBHO 1 1 2 2

=-g~Ho

em

yl ib

ra ry

Thus energy for + 1/2 spin (a-spin) linearly increases with the field Ho while the energy for -1/2 spin (f3-spin) linearly decreases with the field H o (Fig. 12.3.1.1). For the a-spin, magnetic moment arising from the spinning motion of the electron has an unfavourable orientation (i.e. north pole of the magnetic field produced by the spinning electron comes nearer to the north pole of the external field). On the other hand, for the ~-spin, the magnetic moment arising from the spinning motion has a favourable orientation in the external magnetic field. MJ = +...L 2

""~ ------t::::",,hv = g~Ho

ch

(Free ion

..!2

""

th e

J=

al

state)

~-----.~Ho

e/

MJ =-....:!-. 2

ms

=+

i'

a-spin (t)

(E = + .1 g~Ho)

er

e

t.m

2

H

r

k

ms = -

(t)

1

lic C

No field

i '~-spin

(E = -"2 g~Ho) Ho-------+~

Fig. 12.3.1.1 Variation of electron spin energy with the externally applied magnetic field strength and ESR transition (selection rule: ~ms =±1). Inset represents splitting of the state J =1/2 by a magnetic field and ESR transition (selection rule: ~J = ±1); here J is assumed to be a good quantum number (see Vol. 1).

Note: For a proton, the lower energy state corresponds to m/ = +1/2 while the higher energy state corresponds to m/ = -1/2. This difference for the electron and proton arises for their opposite charge bearing properties. • Energy difference and frequency of ESR transition: The mechanism of ESR transition is similar to that of PMR transition. The spinning magnetic moment vector of an electron precesses around the magnetic field. ESR transition occurs when the frequency of the oscillator electromagnetic radiation matches with the precessional frequency. The energy difference (~E) for the two states characterised by + 1/2 and -1/2 is given by: ~ = E 2 - E 1 = g~BHo = g~Ho, (denoting ~B by ~).

1873

SPECTROSCOPIC METHODS AND OTHER PHYSICAL METHODS IN CHEMISTRY

24

g~Ho h

2x9.274x10- JT- I H o 10 -I -I 34 =2.86x10 s T xHo 6.26 x 10- Js

For~=hv,orv=--=

For, H o = 0.5 T (i.e. 5 x 103 G), V = (2.86 X 1010 x 0.5) Hz = 1.43 X 10 10 Hz Generally, the ESR frequency lies in the range 3 x 106 to 3 X 10 10 Hz (microwave region of electromagnetic spectrum). The higher ESR frequency compared to that of PMR frequency is due to the smaller mass of an electron (cf. Sec. 12.2.1) (cf. magnetic moment values, JlN and JlB values, Sec. 12.2.1). • Method of ESR studies: As in the case of PMR studies, it can be done in two ways: varying the

yl ib

ra ry

magnetic field strength for a fixed oscillator frequency or varying the oscillator frequency for a fixed field strength. As in the case of PMR studies, in the ESR studies, generally magnetic field is varied for a fixed oscillator frequency. There are two types of ESR instruments-X- and Q-band spectrometers. In the X-band spectrometer, the operating microwave frequency is around 9400 MHz

th e

al

ch

em

and the magnetic field around 3000 gauss is varied. In the Q-band spectrometer, the operating frequency is around 35,000 MHz and the magnetic field around 12,500 gauss is varied. Obviously, the ESR spectrometer at higher operatingfrequency (i.e. Q-band) is more sensitive. The higher operating frequency requires the higher magnetic field for the ESR transition. The higher field makes the energy difference (~ higher, i.e. the Boltzmann population density ratio (ntln2) becomes higher at the higher field. It enhances the intensity of the ESR signal (cf. intensity of an NMR signal at higher field, Sec. 12.2.2).

Energy change (dE) in ESR and PMR transitions (cf. Sec. 12.2.1)

V pMR

e/

=

= ge~eHO = gp~pHo

ge = 2,

gp~pHo; h

21

""

~e = 9.27 X 10-21 erg a-I

gp =5.5854, .)

~p = 5.05 X 10-24 erg a-I

660

k

V pMR

h

2 x 9.27 X 105.5854 X 5.05 X 10-24

H

V ESR

er

e

PMR:!i.E = hv = gpfJ/fo i.e.

ge~eHo ;

t.m

ESR: !i.E = hv = g.f3po i.e. v ESR =

C

lic

Thus for a particular magnetic field, resonance frequency for an electron is about 660 times higher than that for a proton and VPMR lies in the radio-frequency region while VESR lies in the

microwave region. • Intensity of an ESR signal: The relative distribution of the electrons between the two electronic spin energy states is determined by Boltzmann law. 2

n n

(

+~) =

(_~) l

exp (- liE

kBT

J= exp (- g~Ho J

liE (for ESR) is much larger than

kBT

~

(for PMR) i.e.

population density at the lower level (n l nl -;;; = population density at the higher level' -;;;

J ESR»

(n l

J

-;;; PMR

because ~E (ESR) »

~ (PMR)

1874

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

~

~ c::

o

Absorption curve

e

E-

o

er

en

~

Shoulder

..-J

lic

k

H


V1). The frequency of the corresponding forbidden transition (~Ms = ±2) is (v 1 + v2).

Ms = +1

E+ 1 =8 + g~Ho

Ms = ±1

~-I---

= ±2 ~E= hv

~Ms

= ±1

~E

=hv

.......----~--t--------.----_-

Ms = 0 Eo =0 ~E = hv

(Zero field splitting,

independent of Ho)

C

lic

k

H

er

e

v (microwave or - - hv » ESR frequency) M =0 s

~Ms

Ms =-1 E_ 1 = 8 - gJ3H o

Zero-field splitting removes the spin degeneracy

-+ Ho (b) Moderate zero-field splitting

Fig. 12.3.3.1 Zero-field splitting and ESR transitions for the triplet state (S = I, Ms = 0, ±I). (b) Moderate zero-field splitting (8); two allowed ESR transitions (tiMs =±I) at different fields for a fixed operating frequency producing two strong ESR signals; in addition to these two intense ESR signals, the forbidden transition (tiMs = ±2 i.e. transition between Ms = + 1 and Ms = -I) producir..g a weak signal.

1882 n

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

=2 (i.e., S =1, Ms =±1, 0): The energy states are: M s =±1 ) M s = 0 (Fig. 12.3.3.1b, c)

n = 3

(.

3

3 1)

3 1 , l.e.,S=-,M s =±-,±-: The energy states are: Ms=±-)Ms =±- (Kramers 2 2 2 2 2

degeneracy) (cf. Fig. 12.3.3.2). n

= 4 (i.e., S = 2, M s = ±2, ±1, 0):

The energy states are: M s

= ±2

) Ms

= ±1

) Ms

= 0 (cf.

Fig. 12.3.4.2). n

. 5 5 3 1) =5 ( I.e., S =-, M s =±-, ±-, ±- : The energy states are: 2 222

Ms

=±-5 > M s =±-3 > M s =±-1 2

2

2

(Kramers' degeneracy) (cf. Fig. 12.3.3.3).

em

yl ib

ra ry

Thus for, n =even, except for M s =0, all other energy states are doubly spin degenerate. For n =odd (>1), all the energy states are doubly spin degenerate (Kramers' degeneracy).

Ms =±1

ch

~Ms

Ms =+1

8

Ms =-1

t.m

e/

v (microwave or ESR frequency) hv«

th e

al

= ±2 (Forbidden transition)

lic

k

H

er

e

(Zero-field splitting independent of Ho)

(v 1 and V 2 » microwave frequency) NO ESR Signal

Ms=O

----. Ho (c) Large zero-field splitting

C

Fig. 12.3.3.1 Zero-field splitting (removing the spin degeneracy) and its effect on ESR transitions for a triplet state (i.e. S = 1, Ms =~ ±1) (c) Large zero-field splitting (8) and the energy change for the allowed ~s =±1 transitions becomes too large to be observed in the microwave region; only the forbidden transition (tJMs = ±2) may produce a weak ESR signal.

On application of an external magnetic field, all the doubly degenerate spin states will undergo further splitting. (iv) ESR transitions for S = 1 and M s = ±1, 0: For the Ni ll (t2~ei), V1II(tfg), Fe(VI) (tP) (i.e. FeOlin K2Cr04 host lattice), dimeric complexes of CUll (cP) and oxovanadium(IV) (d l ), the allowed ESR transitions (~s = ± 1) are: M s = -1 ---7 M s = 0 and M s = 0 ---7 M s = +1 (cf. Fig 12.3.3.1b) Besides these two allowed ESR transitions, there is a forbidden ESR transition (AMs = 2) i.e. M s =-1 ---7 M s =+1 These ESR fine peaks may undergo further hyperfine and superhyperfine splitting depending on the condition. This aspect will be illustrated later.

1883

SPECTROSCOPIC METHODS AND OTHER PHYSICAL METHODS IN CHEMISTRY

±.3. , ±1. 2

2

ch

em

yl ib

ra ry

Ms =

al

Ho

th e

(a) One allowed ESR transition at a fixed operating frequency (v)

t.m

e/

Fig. 12.3.3.2 (a) No zero-field splitting and one ESR signal for S

",'

~

",'"

8DQo

>.

en Co C

w

4

T2g

t

k

(~---------,

H

",'"

" "",, ,, , "

lic

(t~g)

",'"

C

F

2

er

",'" ",'" ",'"

4

Ms=+l

T1g

e

4

'" ,....

3 MS =±2

10DQo (4

A29 )

S=~

'

,.'"

",'"

.....

,

8

..... .....

1 MS =±2 (Tetragonal field)

(Zero-field splitting and Zero-field splitting removing the spin degeneracy

(b) Three allowed

I

I

I

I

I

I

I

I

Kramers' degenracy)

\

\

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

+

i

~Ms = ±1

Ms =+l 2

\

2

(Oh crystal field)

3

= 2"

,.~,.,.

,.,." ,."

,.,."

\', \

\

\

" "

\

\

\

\

\

Ms =_l

I I I I I

2

"

\

\

\

\

\

= ±1

Ms=_l \

f I I

~Ms \

hvs

= ±1

~Ms

,,~~

2

3 E = 8 + 2g~Ho

I

f I I I I I I I

+

1 E =2g~Ho hV2

1 E = -2g~Ho hV 1 E = 8 _lg~Ho

2

(~Ms

= ±1) ESR transitions (v 1, v2, v s) at a fixed field

1884

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

Ms=+~ 2

Note: • 8 is called zero-field splitting or tetragonal splitting parameter: If hv « 8, (v = operating frequency in the microwave region) then the transition, M s =

/

--;-", 8 '--&_-
M s =±2" >M s =±2"

There are five unpaired electron

It leads to five allowed ESR transitions (i.e. 5 fine components; Fig. 12.3.3.3a, c) following the

= ±1. 3

5

3

1

= -2~Ms =-2"; M s = -2"~Ms =-2"; 133

5

=+2"; Ms =+2"~Ms =+2"

1

1

= -2~Ms =+2";

em

M s =+2~Ms

Ms

ra ry

Ms

~s

yl ib

selection rule

t.m

e/

th e

al

ch

The high spin (octahedral) d 5 system gives the 6A 1g ground state which is orbitally nondegenerate. This state can mix with the excited 4T 19 state (orbitally degenerate and it can be split by crystal field)

~

El Q)

W

hv s : I I I

+~ 2

er

e

v (ESR frequency)

c:

I I I

S6

l

k

(d

S

lic

6

H

(0 1 «hv)

)

C

(A19)

3

Ms =±"2

Ms=±~ 20

1

+..1 2

---I----IC

-_...Ioo---foII~

(Weak zero-field splitting)

-~ 2

~Ms

=± 1

-..§. 2

---+

Ho

(a) 5 allowed (~Ms = ± 1) ESR transitions (denoted by solid arrows) at a fixed operating frequency (v); 5 allowed ESR transitions (denoted by dotted arrows) at a fixed field. S (Small zero-field splitting i.e. 81 « hv; weak tetragonal field for the high spin d system)

1886

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

~

e> (J) c:

v (ESR frequency)

W

ra ry

(8 1 »hv)

-~ 2

+1-

yl ib

--M-s-=...!+~_-:;-1 ----lr-===:====-_~h~v~

_ _

(gil = 2, g-1- = 6)

1 2

em

- 2 (Strong zero-field splitting)

ch

Ho

(b) Large zero-field splitting (8 1 » hv) and one ESR signal noted between the

+i -i and

states (i.e. S' =

S

i);

Strong tetragonal field for high spin

al

Ms =

t.m

e/

th e

d system) (cf. Fig. 12.3.7.15) for splitting in rhombic symmetry)

C

lic

k

H

er

e

Hyperfine structure transitions

(Zero-field splitting) Kramers' degeneracy; (Small zorofield splitting)

Hyperfine structure transitions

(8 1 «hv) (Hyperfine splitting) (c) 5 allowed ESR transitions (v 1 , v2 , v 3 ,

V4

and vs ) at a fixed field and hyperfine

splitting of each ESR transition by Mn (I

1)

=

1887

SPECTROSCOPIC METHODS AND OTHER PHYSICAL METHODS IN CHEMISTRY

Mn(lI)

/

L..--y---J

1

L..--y---J

'1

~'l"

~

L..--y---J

(5-1Ine peaks)

L..--y---J

~

Most intense signal

llII

Decreasing intensify

Decreasing intensify

3

4

5

6

~

Ho

(e) 6 hyperfine components of an ESR peak of high spin Mn(lI)

C

lic

k

H

er

e

t.m

e/

th e

al

ch

em

2

(I = -%-)

yl ib

(d) Hyperfine splitting of 5 ESR signals by Mn

ra ry

------

_--------------======::::::::;:/I·' g.l) indicates the tetragonal distortion placing the unpaired electron

in the d x2 _ y2 orbital. Type 2 Cu(II) present in Cu, Zn-SOD (superoxide dismutase) maintains a dis-

torted 5-coordinate geometry and it allows the mixing between the d Z2 and d X2 _ y2 orbitals to some extent. It introduces some sort of anisotropy in the gl. components. (iii) ESR spectra of CUA (binuclear copper-core) in nitrous oxide reductase and cytochrome c

oxidase: In the oxidised form, the binuclear copper core shows S =! (i.e. ESR active). In the reduced 2

(s =~) ,

The oxidised form of CUA gives an ESR signal which is

yl ib

and Cu(I) (d lO ) centres are coupled

ra ry

form constituted by 2Cu(l) (dID) centres, it is ESR inactive (i.e. S = 0). In the oxidised form, Cu(ll) (tP)

split into weak 7 hyperfine components (observed under high resolution) by two equivalent

(I = %) centres (2 x 2 x %+ 1 = 7)' This equivalent character of the Cu-sites (evidenced by

em

63,65 eu

ch

the number of hyperfine components) suggests the mixed valence species.

th e

al

The very weak hyperfine splitting indicates that the metal centres are bonded to the ligands in a highly covalent fashion. It may allow the delocalisation of the unpaired electron to the ligand moiety. It may also allow the electron transfer interaction with the Cys-S moiety, i.e. CUll - SR H CuI_·SR

t.m

e/

leading to the accumulation of the unpaired electron on sulfur. In fact, under low resolution, the ESR signal of CUA (oxidised) resembles the signal (without any hyperfine splitting) of an organic free radical. It may be noted that the weak hyperfine splitting also occurs for Type I copper(II).

er

e

B. Fe-S proteins (ferredoxins): In rubredoxin (Rb), iron is tetrahedrally coordinated by four

(d

5

,

S=

%) and it

H

cysteine-S sites and the geometry is distorted. The oxidised form contains Fe(III)

C

lic

k

is EPR active but hyperfine splitting is insignificant due to the poor abundance of 57Pe (cf. 57pe, I = 1/2; natural abundance - 2.3%). At low temperature, it gives two characteristic peaks (Pig. 12.3.7.14) at geff z 4.3 (strong signal) and geff Z 9.5 (single peak) which correspond to the EPR transitions within the first excited and ground state Kramers' doublets repectively. These Kramers' doublets (for S = 512), arise in a rhombic symmetry (Fig. 12.3.7.15). The separation due to zero-field splitting between the Kramers' doublets is too large to observe the ESR transition between the different doublets. However, at most temperatures, all the three Kramers doublets in rhombic symmetry for S =~ may be significantly 2 populated to observe the ESR transitions within the each doublet. This large rhombic zero-field splitting is common for the distorted tetrahedral system with S = ~ . 2

In contrast to rubredoxin (oxidisedform, S =~), for other Fe-S proteins (to be discussed later), 2 they are characterised by S' =! with the g-values close to 2.0. 2

1932

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

S(cys)

l-

1 111

Fe

!

(CYS)S/ \S(CYS) i. e. Fe(S-cys)4(cys)

9 = 2.05

gaff::::: 9.5

ra ry

9aff::::: 4 .3 (Strong signal)

yl ib

Rbox

------------+~Ho

(a)

em

(b)

ch

Fig. 12.3.7.14 Qualitative representation of the EPR spectra (recorded at low temperature) of Fe-S proteins: (a) Oxidised form of rubredoxin; (b) Reduced form of Fe2S2 ferredoxin.

8

»hV

ESR ;

8 >/ ksT (depending on T)

H

er

e

t.m

e/

th e

al

Note: For Rbox, geff (isotropic) ::::: 4.3 indicates the signal that arises from the ESR transition within the middle doublet, i.e. first excited Kramers' doublet. But geff(anisotropic) ::::: 9.5 is expected when the applied field (Ho) is parallel to the y and z axes (cf. gy::::: 9.67, bottom doublet; gz::::: 9.67, topmost doublet; see Fig. 21.3.7.15). However, at low temperature, the topmost doublet is not populated sufficiently to show the ESR signal. Thus the observed geff ::::: 9.5 reasonably corresponds to g y of the bottom doublet.

lic

k

gx = 0.86, gy = 0.6, gz = 9.67 (anisotropic transitions)

C

gx = 4.3, gy =4.3, gz = 4.3 (isotropic transitions)

gx = 0.86, gy = 9.67, gz = 0.6 (anisotropic transitions) ~--------------.Ho

o

Fig. 12.3.7.15 Splitting of three Kramers' doublets for S = 5/2 in rhombic symmetry (see Fig. 12.3.3.3b for splitting in a tetragonal field) by the effect of zero-field splitting (0) and magnetic field. The transitions within each Kramers' doublet are allowed. The g-values within the three Kramers' doublets are indicated.

Here it is worth noting that the reduced form of Rb possesses 4 unpaired electrons (Fell, tf, S = 2) but it is EPR silent (cf. large zero-field splitting for S = 2; Fig. 12.3.4.2).

1933

SPECTROSCOPIC METHODS AND OTHER PHYSICAL METHODS IN CHEMISTRY

The FezSz-ferredoxins (having two ll2-sulfide) is ESR active Fe(III)

(d

5

,S =

(s =±)

in its reduced form in which

%) ,and Fe(II)(tf', S =2) are antiferromagnetically coupled. The oxidised form is ESR

inactive (S = 0). In the oxidised fOlTI the two Fe(II1) centres ( S =

%) are antiferromagnetically coupled

(i.e. S = 0).

The

Fe~3 ferredoxin (void cubane structure) is EPR active in its oxidised form

(s =±'

3Fe

III

).

±)

and the 2e- reduced form (i.e. Fdred) constituted by the coupled

(s = ±).

The intermediate form i.e. 2Fe(lII) + 2Fe II (i.e.

em

IFelII and 3FeII centres is also EPR active

yl ib

and 1FeII centres is EPR active ( S =

ra ry

For the Fe4S4 protein (cubane structure), the oxidised form (HiPIPox) constituted by the coupled 3FeIII

HiPIPred or Fdox ) is EPR inactive (S = 0).

1.89, 1.95, 2.05 (reduced form)

4Fe-ferredoxin (4Fe-4S)

1.97, 2.00, 2.02 (oxidised form)

2.04, 2.04, 2.12 (oxidised form)

al

-4, -9.5 (oxidised form)

3Fe-ferredoxin (3Fe-4S)

e/

th e

2Fe-ferredoxin (2Fe-2S)

t.m

g-values:

Rubredoxin (Rb) (lFe-OS)

ch

The g-values (::::: 2.0) of the common Fe-S proteins are given below (from the ESR spectra recorded at low temperature)

3FellI , 1Fell,

e lic

Signal)

=

~)

(HiPIPox)

(Reduced form) g

er H

k

(Rhombic EPR

(s

~

1.88, 1.92, 2.06

(s =~)

(Rhombic EPR

(Rhombic EPR

IFeIII,3Fe ll ,

Signal)

Signal)

(Rhombic EPR Signal)

(Fd.-oo)

C

The condition, gx =t= gy =t= gz' indicates the rhombic symmetry (cf Sec. 12.3.11). The structural details of Fe-S proteins are given in the author's book, Bioinorganic Chemistry.

12.3.8 ESR Spectra: Magnetically Concentrated

vs. Magnetically Diluted Complexes

The nature of magnetic exchange interaction can be understood by following the ESR signal. Let us take the case of S = 1/2 (e.g. 3d l , V0 2+; 3cf, Cu 2+; etc.). If the binuclear complex is taken into consideration then coupling of the two S = 1/2 centres (i.e. Sl = S2 =1/2) leads to the resultant values: S = 0 . (singlet) and S = 1(triplet). The singlet state (i.e. S = 0) is the ground state for the antiferromagnetic exchange interaction while the triplet state (S = 1) is the ground state for the ferromagnetic exchange interaction (cf Chapter 8). Antiferromagnetic exchange: Ground state (S = 0), excited state (S = 1). Ferromagnetic exchange: Ground state (S = 1), excited state (S = 0). The population density (determined by Boltzmann distribution) at each state is determined by the relative magnitudes of the energy difference (t1£) between the states and available thermal energy (kBT). dE can be expressed in terms of the exchange integral (1).

1934

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

(I =~' 2 x 2 x ~ + 1= 15) (cf Fig. 12.3.7.7). For the dimeric

yl ib

components by two equivalent V-centres

ra ry

The singlet state (bearing no unpaired electron) is ESR inactive while the triplet state is ESR active. Thus, when an antiferromagnetic exchange prevails, with the increase of temperature, the population density at the triplet state (i.e. excited state bearing the unpaired electrons) increases and consequently, intensity of the ESR signal increases. On the other hand, when the ferromagnetic exchange interaction prevails, with the increase of temperature, the population density at the singlet state increases and consequently, intensity of the ESR signal decreases. 8 1 =8 2 =1/2: By taking into the consideration of zero-field splitting, two allowed ESR transitions (liMs = ±1) are expected along with a forbidden ESR transition (1ll1s = ±2) of weak intensity (cf. Fig. 12.3.31b). Existence of this forbidden transition supports the interaction between ihe two centres of 8. = 8 2 = 1/2. In fact, in the dimeric complexes of copper(II) and dimeric complexes of oxidovanadium(IV), the characteristic forbidden transition (l!.Ms = ±2) with a poor intensity is noticed. For the dimeric oxidovanadium(IV) complexes, this weak ESR signal is split into 15 hyperfine

(I =%, 2X2X%+1=7) (see Fig. 12.3.7.8).

ch

components

em

copper(II) complexes, the said weak ESR signal is split by two equivalent Cu-centres into 7 hyperfine

al

12.3.9 Factors Affecting the g-value (ct. Secs. 8.15,8.17,8.18)

th e

For an unpaired electron residing in a gaseous atom or ion obeying the Russel-Saunder Coupling (i.e. spin-orbit coupling constant is large), the g-value is given by:

t.m

e/

=1+ J (J + 1) + S(S + 1) g

L(L + 1)

2J(J + 1)

e

where the symbols bear the significance as usual. For the halogen atoms, the ground state term is 1 3 4 and J =-. By using the above equation, we get g =- which is experimentally 2 2 3 verified. For a free electron, S = 1/2, L = 0, (i.e. no orbital contribution to the magnetic moment) J = 1/2 and it leads to g = 2.0. Taking into the consideration of the relativistic correction, the g-value for a free electron becomes 2.0023. When the unpaired electron is placed in a chemical environment (e.g. in a transition metal complex), the orbital motion of the electron is perturbed. In other words, in a chemical environment, the orbital degeneracy which exists in a gaseous state or in a spherical field is, at least, partly destroyed (i.e. orbital motion is partly quenched). Specially, the low symmetry crystal field and Jahn- Teller distortions are extremely important in removing the orbital degeneracy. This is why, the orbital contribution to the magnetic moment is partially quenched in a crystal field. On the other hand, the spin-orbit coupling phenomenon (leading to some orbital contribution to J.!obs) tends to restore the orbital degeneracy to some extent. Thus the crystal field effect destroying the orbital degeneracy is opposed by the spinorbit coupling effect. Because of these two opposing contributions, the orbital degeneracy is only partly removed and a partial contribution of the orbital magnetic moment is retained. Thus the g-value is different from 2.0023. In absence of spin-orbit coupling, the expected g-value is 2.0023. The unpaired electron in a free radical is not generally localised or confined in a particular orbital, i. e. it can freely move over the orbitals encompassing the whole molecule. In this sense, the electron behaves like a free electron in space (i.e. L = 0). For most of the free radicals, the orbital

k

H

er

= 1, S =-

lic

i.e. L

C

3P312

1935

SPECTROSCOPIC METHODS AND OTHER PHYSICAL METHODS IN CHEMISTRY

H

er

e

t.m

e/

th e

al

ch

em

yl ib

ra ry

contribution is very small because of the lower molecular symmetry or the loss of degeneracy of the energy levels by Jahn-Teller distortion. Moreover, the spin-orbit coupling is not also important for the free radicals. Consequently, the isotropic g-value for the free radicals is generally close 2.0 i.e. free electron g-value. Such typical organic free radicals show no hyperfine interaction indicating the complete delocalisation of the electron. The unpaired electron in a free radical is not localised in a particular orbital but in a transition metal complex, the unpaired electron is localised in a particular orbital. In different transition metal complexes, depending on the different governing factors, g can have different values. These different values arise for the different degrees of crystal field effect and spin-orbit coupling effect. For the both cubic symmetry producing the orbitally nondegenerate ground state (e.g. high spin d 5, 6S, having the 6A 1g ground state) and highly distorted symmetry (where all the orbital contributions to the magnetic moment are quenched), g value becomes close to 2.0. For the rare earth metal ions, the spin-orbit coupling interaction is much stronger (i.e. J is a good quantum number) than the crystal field effect on the (n- 2)f orbitals which are deeply seated. This is why, the lanthanide ions behave like the free ions and the g-value can be calculated by using the given relation. For the transition metal ions (specially the 1st transition series), the crystal field effect on the (n-l)d orbitals is more important than the spin-orbit coupling interaction (i.e. J is no longer a good quantum number). In such cases, the orbital degeneracy is lifted by the predominant crystal field effect. This is why, the g value is very close to 2.0 and the magnetic moment is very often given by the spinonly value. But for the systems where the orbital angular momentum exists to some extent as in the cases where the ground state level is designated by T(g), the spin-orbit coupling operates and the g-value deviates from the spin-only g-value of 2.0. For the systems like Fe3+ (high spin, L = 0, J = S, 6S) and Mn2+ (high spin, 6S) which are having the orbitally nondegenerate ground states, the g-value is very close to that of a free electron. To measure the quantitative effects of the spin-orbit coupling process and the crystal field effect on the g- value, we are to recall the following relations.

2 J=ge (1- lODq a'Ak J, ge =

gspin-only

=2.0 (cf.

Sec. 8.17)

C

lic

k

a'Ak2 geff =gspin-only ( 1- lODq

. - (1- 4k AJ=2(1_ 4k AJ

For A 2(g) ground state. geff - ge

2

2

10Dq

2k2'A For E(g) ground state: geff = ge ( 1- lODq

10Dq

2 J=2 (1- lODq 2k 'A J

k = orbital reduction factor; A = spin-orbit coupling constant A = +ve (for the d l -4 systems), A = -ve (for the tf'-9 systems).

It indicates that geff tends to be ge for the higher crystal field splitting (i.e. higher 10Dq value) and the smaller spin-orbit coupling constant. For the A and E ground states, the spin-orbit coupling mixes the ground state and the excited T state which retains the orbital angular momentum to some extent. For the Ni(II) octahedral complexes, the ground state is 3A zg which mixes with the excited 3TZg state through the spin-orbit coupling mechanism and geff is given by:

1936

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

g iff e

4k2A J 8A = 2 ( 1- - ::::: 2 - - 10Dq

.

10Dq'

In [Ni(OH2)6]2+, A = -270 cm- l , 10Dq spectrum) and these lead to gejf = 2.25

= 8,500

assumIng k ::::: 1

cm- l (estimated from the electronic absorption

12.3.10 Determination of the g-value

al

@N-N

ch

em

~...02N

yl ib

ra ry

The g-value can be obtained from the condition of ESR transition (assuming no hyperfine and superhyperfine splitting) i.e. hv =g~Ho or g =hV/~Ho, if the microwave frequency of the ESR transition is known. In fact, an ESR spectrometer works at a particular operating frequency (called probe frequency) and the magnetic field is varied to find the field strength at which the resonance works. If the microwave frequency is not known accurately, then the spectrum is calibrated by using a reference sample whose g value is accurately known. For this purpose, 2, 2-diphenyl-l-picrylhydrazyl (DPPH) radical having g = 2.0036 is used.

e/

th e

02N

t.m

(DPPH)

= gDPPH

er

gsample

e

The g-value of the sample is calculated by using the following relationship.

(1- ~)

= 200036(

1- ~)

C

lic

k

H

where ~H gives the difference between the resonating fields required for the sample and DPPH radical. The positive value of All indicates the sample to resonate at a higher field while the negative value of All indicates the sample to resonate at a lower field. ~H is a small number compared to the resonance magnetic field H (obtained from the range used for the ESR study) and consequently the approximate value of H does not significantly affect the g-value. The negative value of ~H indicates gsample ) gOPPH while the positive value of ~H indicates gsample ( gOPPH' In actual practice, the standard sample DPPH is placed along with the unknown sample in the same chamber of a dual cavity cell to record the ESR signals of both the standard and unknown sample. From their ESR signals, the field separation ~H is obtained. The standard sample DPPH can also be used to estimate the number of unpaired electrons participating in producing the ESR signal. The peak area covered by either the absorption or derivative

curve is proportional to the number of unpaired electrons responsible for the ESR transition. DPPH contains one unpaired electron per molecule. It leads to 1.53 x 1021 unpaired electrons per gram of DPPH. K2[NO(S03)2] (Fremy's salt, bearing the nitrosyldisulfonate radical, g = 2.0057) can also be used as an standard substance to determine the g-value of the unknown substances. To determine the g-value of other free radicals, DPPH or K 2[NO(S03)2] cannot be llsed as the standard sample because for most of the free radicals, the g value is close to 2.0. Consequently, ~H

1937

SPECTROSCOPIC METHODS AND OTHER PHYSICAL METHODS IN CHEMISTRY

cannot be determined accurately. In such cases, a trace amount of Cr(III) entrapped in a tiny chip of ruby crystal is used as the internal standard. This Cr(III)-standard sample shows a strong ESR signal . having the g value 1.4.

12.3.11 Anisotropic Behaviour of 9 and Anisotropy in Hyperfine Interaction

yl ib

ra ry

The g-value depends on the direction of the paramagnetic species with respect to the direction of the applied magnetic field. When the paramagnetic substance is rotated to all directions with an equal probability, the obtained g-value is averaged over all the possible orientations and it is denoted by gave It occurs so lvhen the measurement is carried out in the solution or gas phase where free rotation ofthe species is not restricted. However, if the paramagnetic centre lies in a symmetric cubic field (e.g. octahedral or tetrahedral site), the g-value does not depend on the direction, i.e. g is isotropic. The isotropic nature exists in the perfectly octahedral complexes. But for the crystal field of lower symmetry, the g-value is dependent on the direction and g becomes anisotropic.

gil

gz 9x

ch

9.1

(Rhombic spectrum)

em

(Axial spectrum)

(Isotropic spectrum)

th e

al

911) 9.1

t.m

e/

Fig. 12.3.11.1 Qualitative representation of the ESR spectra of a frozen solution or a polycrystalline solid state (for S =

±).

C

lic

k

H

er

e

In the anisotropic systems, the resultant g can be expressed in terms of the components gx' gy and gz which are mutually perpendicular. The z-direction is defined along the highest fold rotation axis (en). Then gz is consider to be gil (i.e. gz = gil) and it is obtained when the z-axis is parallel with the magnetic field direction. The g-values along with the x and y axes are gx and gy respectively. In a cubic crystal field (as in the perfectly octahedral complexes), all the metal-ligand bond lengths are the same along the three Cartesian axes and it leads to gx = gy = gz. In such cases, g is said to be isotropic. In a tetragonal symmetry (D4h), the metal-ligand distances along the x- and y-axes are the same but different from the metal-ligand distance along the z-axis. Thus the g-value shows an anisotropic behaviour i.e. gz = gil -:t gx = gy = gl.. gl. is obtained when the external magnetic field is perpendicular to the z-axis. Thus the notations, II and 1. refer to the directions along and perpendicular to the C4 axis in D4h symmetry (i.e. tetragonal symmetry). Similarly, lilies along the C3 axis and 1. lies perpendicular to the C3 axis in D 3h (i.e. trigonal) symmetry. For a complex of rhombic symmetry, the metal-ligand distances along the three Cartesian axes are different and the g value shows an anisotropic behaviour, i.e. gx -:t gy -:t gz. Magnetic moment measurement of a powdered sample determines an average value of g i.e. gave On the other hand, if a single crystal is used for an ESR study, depending on the direction of orientation of the crystal with respect to the magnetic field, the directional g values (i.e. gx' gY' gz) can be determined. For a tetragonal symmetry grms and gay are expressed as follows:

2

1( 2 2 2) ="31(2g.l+ 2 g112)

grms="3gx+gy+gz

1938

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

Similarly, the anisotropy in the hyperfine interaction is averaged to zero when the sample is present in a solution and the isotropic hyperfine splitting constant is denoted b~T A o or A iso . The anisotropy in the hyperfine interaction exists when the sample is present in a polycrystalline state or frozen solution. Then the hyperfine-splitting constant (A) is gives by:

1 Aav = "J(A II + 2A-L). If e is the angle between the direction of the external magnetic field and the z-axis (i.e. highest fold en axis) then the resultant g-value is given by: g;esultant = gn cos 2 e + gi sin 2 e

er

e

t.m

e/

th e

al

ch

em

yl ib

ra ry

The anisotropic behaviour of g indicates the deviation from the cubic crystal field symmetry. A very small distortion can also be determined from the ESR studies which show the inequality for the g- values in different directions. • T(g) ground terms and anisotropic g-values: Spin-orbit coupling and low-symmetry ligand field components (e.g. J.T. distortion) lead to splitting of the T(g) ground state. It makes the g-value highly anisotropic. Here it is worth mentioning that if the distortion is very large (i.e. low symmetry ligand field component is large) and splitting of the T(g) term is larger than Athen the orbital angular momentum is fully quenched. Then it leads to g approximately isotropic and its value is close to 2.0 i.e. free electron value. It may be again mentioned that the spin-orbit coupling and low symmetry ligand field components split the T(g) ground state and this splitting is in the order of 200 cm- l (which is comparable to kBT at room temperature). The energy levels lying about by kBT above the ground level favours the spin-lattice relaxation. This is why, to record the ESR spectrum for the sample having the T (g) ground state, the temperature is to be reduced (so that kBT becomes of a few cm-I ) to the liquid He or N2 temperature. Such a problem does not arise for the A(g) and E(g) ground states for which presence of the energy levels separated from the ground level by the order of kBT is unlikely. In fact, their ESR signals can be recorded at room temperature (cf. Sec. 12.3.2).

k

H

12.3.12 Anisotropic Behaviour of the g-value of the Tetragonally Distorted Copper(lI) Complexes

C

lic

It has been already mentioned that the free electron g-value i.e. ge (= 2.0023 ::::: 2.0) is modified under the influence of spin-orbit coupling. The effective g-values can be mechanically computed with the help of the Magic Pentagon (Fig. 12.3.12.1) for the S

=!

systems.

m,

o

z

n'A

g = ge - M

=2.0023 -

n'A

n'A

2

6 /d

M "" 2.0 - M

2

x -y

2

d

xz

~

2 2

2

6

~ ~

/

where ~ = energy difference between the orbital containing the unpaired electron and the orbital with which it may mix by the spin-orbit coupling; A = spin-orbit coupling constant of the free ion; n is an integer obtained from the magic pentagon (e.g. n = 8 for the interaction between the d

'",

dyz

±1

2

2

and dxy orbitals; n = 6 for

the interaction between the d Z2 and dyz orbitals; cf. Fig. 12.3.12.1). ±2

In the magic pentagon, the three rows represent the orbitals in terms of the ml values i. e. d Z2 for ml

= 0; dxz and d

yZ

for ml

= ± 1;

Fig. 12.3.12.1 Magic pentagon

1939

SPECTROSCOPIC METHODS AND OTHER PHYSICAL METHODS IN CHEMISTRY

dxy and d x 2 _y2 for ml =±2. Application of the magic pentagon will be illustrated for the Cu(II) complexes (d 9 system).

(z-out)

Tetragonal elongation

em

yl ib

(z-in)

- Tetragonal compression

ra ry

Cu(II) (d 9) system can experience both the tetragonal compression (i.e. z-in) and tetragonal elongation (i.e. z-out) distortions depending on the situation.

ch

Fig. 12.3.12.2 Electronic configuration of tP system for z-in and z-out distortion.

al

Tetragonal compression (i.e. z-in): The ninth unpaired electron is in the d z2 orbital (energy order: d z2 > d x2 -y 2, cf. Chapter 3).

e/

th e

For the tetragonal compression, the g-values are:

=2-~

t.m

10Dql.

i.e. gl. ) gil = 2.0 (cf. A = -ve)

er

e

Tetragonal elongation (i.e. z-out): The ninth unpaired electron is in the d x 2 -y 2 orbital (energy

k

H

order: d x 2 -y 2 > d z2 , cf. Chapter 3) For the tetragonal elongation, the g- values are:

C

lic

gz = gil =2

Ed 2 x

8A _ _ =2_ 8A =2-~=2-~ -i

- Ed

dEli

10Dqo

10Dqll

xy

i.e. gil ) g.l ) ge = 2.0 (cf. A = -ve; 10Dq.l) 10Dqll)· Tetragonal compression (i.e. z-in distortion) is rarely found for the Cu(II) complexes but tetragonal elongation (i.e. z-out distortion) is very much common for the Cu(II) complexes. For the Cu(II)-porphyrin complexes, the g-values, gil = gz = 2.70 > gx = gy = g.l = 2.04> ge = 2.0 indicate the z-out distortion. In some cases, the dynamic Jahn-Teller distortion may lead to the time averaged perfect octahedral structure. It is obvious that for high splitting energy (i.e. 10Dq large), g-value approaches to zero. Let us illustrate the effect of crystal structure on the g-value by taking the example of CuSiF6 . 6H20 i.e. [Cu(OH 2)6]SiF6 which is doped as an impurity into the isomorphous diamagnetic host crystal, ZnSiF6 · 6H20 for an ESR study. The ESR spectrum (cf. Fig. 12.3.7.13) at different temperatures gives the following results:

1940

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

gIl =2.45±0.01}

20K g-t = 2.10±0.01 (-253°C)

gIl = 2.220 ±0.005}

90K g-t = 2.230±0.005 (-183°C)

(i.e., anisotropic behaviour of

(i.e., isotropic behavi~ur of

g)

g)

t.m

e/

th e

al

ch

em

yl ib

ra ry

The anisotropic behaviour of g (gil) g.l ) 2.0) at low temperature indicates the tetragonally elongated structure of [CU(OH Z)6]z+. But the isotropic behaviour of g at the higher temperature indicates the undistorted (i.e. cubic symmetry) structure of [CU(OH Z)6]Z+. [CU(OHZ)6]z+ is expected to have a tetragonally distorted structure in terms of Jahn-Teller distortion. The tetragonal distortion (i.e. elongation of the trans-metal-ligand bonds) can occur along the three mutually perpendicular axes with an equal probability in the present case where all the six ligands are identical. If these three distorted forms are frozen out within the ESR time scale, we can expect three ESR signals. At the higher temperature (90 K), one ESR transition (with the hyperfine structure, i.e. 4 lines due to 63CU having 1= 3/2) with an isotropic g-value is noted. It suggests that the three distorted species are in a rapid dynamic equilibrium and it is referred to as the dynamic Jabn-Teller distortion which in turn gives the time averaged perfect octahedral structure of [Cu(OH 2)6]z+. It leads to the crystal field to resonate rapidly among the three possible distorted structures. At the tower temperature (say 20 K), the three distorted forms (1: 1: 1) are frozen out as the available thermal energy is not sufficient to overcome the kinetic barrier for the interconversion process and it leads to the static Jahn-Teller distortion supported by the anisotropic ESR spectrum consisting of three sets of lines. The dynamic J.T. effect is observed in the EPR spectrum of Cu(ll) doped into Zn(II)-Thttons salt (double sulfates M2'(Cu/Zn)(S04)2· 6H zO, having [CU(OH Z)6]Z+). A similar observation of the resonating crystal field (i.e. dynamic Jahn-Teller distortion) at an elevated temperature has been noted for [Cu(bpY)3]z+, [Cu(phen)3]2+, etc.

e

12.3.13 Anisotropic Behaviour of the g-Value for the Tetragonally Distorted Nickel(lI) Complexes

er

is the ground state and 3Tzg is the next higher state for the octahedral complexes of Ni(ll) (cf system). Tetragonal distortion in the Ni(ll) complexes splits the 3Tzg level into the 3Eg (two fold orbital degeneracy) and 3B zg (orbitally nondegenerate) levels (see Fig. 12.3.4.3). The deviation of the g-value from the free electron value (= 2.0) for the A(g) and E(g) ground states occurs due to the interaction with the higher Tz(g) state through the spin-orbit coupling. The g-values for the Az(g) terms are as follows:

C

lic

k

H

3A 2g

2

g,,=g e

2

(1- 10Dqll 4k A.J=2(1_ 10Dqll 4k A.J 2

(1- 4k A, gl. - ge 10Dql.

z

J-- 2(1- 10Dql. 4k A, J.

EPR studies of (NH4)2Ni(S04)z· 6HzO diluted in the diamagnetic host lattice of (NH4)z Zn(S04)Z . 6H zO indicate the nearly isotropic g-value 2.25 (i.e. gil ::::: g.l = 2.25). Using 10Dq = 8,500 cm- I (obtained from the electronic spectrum of [Ni(OH z)6]2+), we get: 2.25 = 2(1- ~), (taking k = 1). It leads to: 10Dq

A = -266 cm- I which is considerably less than the free ion value (= -324 cm- I ). Lowering of the A value due to complexation is quite expected.

1941

SPECTROSCOPIC METHODS AND OTHER PHYSICAL METHODS IN CHEMISTRY

12.3.14 EPR Peak Broadening and Peak Merging and Electron Spin Exchange Rate It has been noted that with the increase of proton exchange rate, the PMR signals undergo broadening and at a certain proton exchange rate, they undergo merging to give a sharp single peak (cf Sec. 12.2.15). Similarly, with the increase of electron spin exchange rate, the EPR hyperfine peaks may undergo broadening (i.e. exchange broadening) and finally merging into a single peak (i.e. exchange

narrowing).

yl ib

ra ry

(i)

ch

em

(ii)

Ho

th e

-----.~

al

(iii)

e/

Fig. 12.3.14.1 EPR spectra of (Me3C)2NO at different concentrations in ethanol at 25°C. (i) 10-4 M (ii) 10-2 M, (iii) 10- 1 M.

t.m

The electron spin exchange process is very common for the free radicals and in solution it is a bimolecular process in which two radicals collide and exchange their electrons. Thus at low

C

lic

k

H

er

e

concentration, the spin exchange process is slow with respect to the EPR time scale and the hyperfine EPR peaks (as in DPPH, nitroxides) are distinctly observed. B.ut at high concentration, the bimolecular spin exchange process becomes quite fast and the hyperfine peaks undergo merging to give a sharp single peak. From the knowledge of EPR peak broadening and peak merging with increase of electron spin exchange rate, the rate constant of the exchange process can be estin1ated. It is illustrated in some representative cases. (i) Di-t-butyl nitroxide, (Me3C)2NO: The unpaired electron giving the ESR signal is expected to undergo hyperfine splitting by 14N (I = 1) to produce 3 (= 2 x 1 x 1 + 1) hyperfine peaks which are noted at very low concentration (10-4 M in ethanol at 25°C). But with the increase of concentration, the three hyperfine peaks merge to give a weighted average single peak (at about .10- 1 M concentration in ethanol). It is illustrated in Fig. 12.13.14.1. The estimated second order rate constant is 7 x 109 dm3 mol- I S-I. (ii) DPPH (diphenylpicrylhydrazyl radical): The EPR signal due to the unpaired electron shows the hyperfine peaks (5 components, splitting by two 14N nuclei, 2 'x 2 x 1 + 1 = 5) in a dilute solution (ca. 10-3 M in xylene) but in a concentrated solution it gives a sharp .singlet peak (Fig. 12.13.14.2). (iii) Electron exchange between naphthalene and naphthalene negative ion: When naphthalene is added to a solution of naphthalene negative ion, the electron exchange between the species leads to broadening of the hyperfine components (25 lines) of the EPR signal (cf Sec. 12.3.7 (7». The estimated second order electron exchange rate constant in THF is 6 x 107 dm3 mol- 1 S-I.

1942

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

-----.~

yl ib

ra ry

(DPPH)

Ho

-----.~

em

(i)

Ho

(ii)

ch

Fig. 12.3.14.2 ESR spectra of DPPH (i) concentrated solution in xylene, (ii) Dilute solution (ca. 10-3 M).

e/

th e

al

Note: The two 14N nuclei to cause the hyperfine splitting are nonequivalent and the ESR spectrum should contain 9 lines: (2 x 1 x I N I + 1) (2 x 1 x IN2 + 1) =3 x 3. The appearance of Slines = 2 x 2 X IN + 1 = 2 x 2 x 1 + 1 instead of 9 lines may be explained in two ways: assuming the nitrogen nuclei to be equivalents (at least under low resolution) or their hyperfine coupling constants (i.e. ANt and A N2) to be comparable. In fact, it has been noted that the ratio ANI: A N2 is close to unity. A similar situation arises for the CH3CH 2 radical where A CH3 / A CH2 ~ 1 (cf Fig. 12.3.5.6).

t.m

12.3.15 Applications of ESR Spectroscopy (i) Structural information: It can provide the information: • regarding the location of the unpaired

C

lic

k

H

er

e

electron; • location of the nuclei to split the signal; • summation of the intensities leads to the evaluation of the number of unpaired electrons; • comparison of the g-value with free electron value providing the relative importance of crystal field effect and spin-orbit coupling effect; • distortion in the complex; • magnetic properties of the species; etc. ESR studies can be made in solutions, in solid powdered forms and in single crystals. In solution or solid powdered forms, all the possible orientations of the paramagnetic sample with respect to the rrlagnetic field are equally probable. The results from the ESR studies in solution or powdered phase can provide the information of the magnetic properties. However, the ESR studies in single crystals can give the details of the symmetries of the complex. Several examples have been already discussed in the previous sections. (ii) McConnell equation and residence probability of the unpaired electron to the magnetic nuclei to cause splitting of the ESR signal: Determination of the hyperfine coupling constant (A) and the use of McConnell equation can calculate the unpaired electron spin density at the immediate vicinity of the nuclei to cause splitting of the signal. This calculation can lead to the mapping of the orbital to house the unpaired electron. (iii) Detection of free radical: When the concentration of free radical is very low, the paramagnetism cannot he detected by a Gouy balance but the ESR studies can detect the free radicals even at extremely low concentrations (e.g. DPPH radical can be detected even when its amount is lO-12 g in the spectrometer). In the quinone-hydroquinone redox couple, existence of the semiquinone as an intermediate has been proved from the ESR studies. This aspect has been already discussed.

1943

SPECTROSCOPIC METHODS AND OTHER PHYSICAL METHODS IN CHEMISTRY

lic

k

H

er

e

t.m

e/

th e

al

ch

em

yl ib

ra ry

(iv) Nitroxide molecules as the spin labels: The long-lived stable nitroxide molecules like TEMPO, TEMPOL (discussed earlier) can attach with the large biomolecules like proteins and membranes at some specific sites which can change the characteristics of the ESR signal of the nitroxides. Thus studies of the ESR signals of these spin labels can provide the information regarding the structural features of the biomolecules. (v) Rate determination of electron exchange reactions by EPR studies: PMR studies can determine the proton exchange rate (ef Sec. 12.2.15), if the process is not too fast. The proton exchange leads to the PMR peak broadening which has been interpreted by considering the uncertainty principle (h~v~t:::::: h). Similarly, the electron spin exchange process i~ads to the broadening of the hyperfine components of the ESR signals. This allows the rate determination of the electron exchange process (ef Sec. 12.13.14). (vi) Structural information of the inorganic compounds from the ESR studies: This aspect has been already illustrated by taking some representative examples. (vii) Determination of oxidation state of a metal centre: Cu(ll) (cP) is EPR active while Cu(l) (d lO ) is EPR inactive. This technique has been utilised in determining the oxidation states of copper in different copper proteins. (viii) CFT vs ACFT or MOT: Superhyperfine splitting of the ESR signal by the magnetic nuclei present in the ligands supports the fact that the metal d-electron is delocalised over the ligands through the metal-ligand orbital overlap interaction. It indicates that the ideal CFT is not applicable in the metal complexes. This aspect has been already illustrated. (ix) Double resonance technique: Double resonance technique has been found important in NMR spectroscopy. Similarly, the double resonance techniques like ENDOR (electron-nucleus double resonance), and ELDOR (electron-electron double resonance) are found important in EPR spectroscopy. In the ENDOR technique, one ESR transition is followed when the nuclear spin transition is made saturated while in the ELDOR technique, one ESR transition is followed when another electron spin transition is made saturated. Like the nuclear overhouser effect of NMR spectroscopy, here also the enhanced intensity of the signal is also gained. The details of the ENDOR and ELDOR techniques are not discussed here. (x) Characterisation of metalloproteins: The iron and copper proteins are very often found ESR active. Their ESR studies can help us to characterise their structural features.

C

12.4 MOSSBAUER SPECTROSCOPY 12.4.1 Isomeric Nuclides (of an Element) Differing in Energy States: Nuclear Transition Energy in the y-Region When the nuclide jumps down from the excited state to its ground state, it emits y-radiation which may be absorbed by a nuclide (generally of the same element) at the ground state to get excited to the upper energy level. The energy difference (~) between the nuclear ground state and excited state depends upon the chemical environment around the nucleus of the same element. If the source, i.e. enlitter (at the excited state) and the absorber, i.e. sample (at the ground state) are of the same element then the chemical environment induced energy changes at the sample nuclide are very much small but quite important to understand the chemical environment. In the recoilless transition, adjustnlent of the Doppler shift to the emitted y-radiation can allow it to be rightly absorbed by the sample nuclide. The direction and magnitude of the Doppler shift required to bring about the absorption of the emitted y-radiation by the sample nuclide gives us the information regarding the chemical environment around the sample nuclide. This y-ray spectroscopy (i.e. nuclear transition

1944

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

occurring in the y-region) is commonly known as the Mossbauer (MB) spectroscopy to tribute the discoverer who was honoured Nobel Prize in 1961 for this work.

12.4.2 Principle of Mossbauer Spectroscopy: Condition of Recoilless Emission and Absorption of y-Ray through the Adjustment of Doppler Shift

ch

of the nucleus. • Recoil energy (ER): A photon of very high energy (v

em

yl ib

ra ry

(a) Nuclear resonance absorption: When a nucleus jumps down from its excited state, it emits a y-radiation which is to be absorbed by another nucleus for its excitation. If the energy of the emitted y-radiation matches with the energy required for the nuclear energy transition of the sample nucleus then, the condition of nuclear resonance is attained i. e. Ey-source (energy of the emitted y-rays from a source nucleus) = Ey-sample (energy of the y-ray required for the nuclear transition in the sample nucleus) (b) Energy of the emitted y-radiation from a source nucleus: If the energy difference between the excited (Eex ) and ground (Egd ) nuclear states is dE, then the energy of the emitted radiation (for the transition from the excited state to the ground state) is given by: Ey (source) = dE + ED - ER ; dE = Eex -- E gd = Eo (say) = Eo + ED - ER ~ Eo - ER , E R » ED where ED = Doppler shift (due to the translational motion of the emitter nucleus), ER = recoil energy

al

h

= i'

1018 Hz i.e. y-photon) is associated with

de-Broglie relationship). Thus when a y-photon comes out from a

th e

a large momentum (cf. p

z

t.m

e/

nucleus, to maintain the principle of conservation of momentum, the nucleus will recoil (i. e. movement in the opposite direction). The similar phenomenon is experienced when a bullet comes out from a gun. Thus the recoil energy represents the kinetic energy of the recoil nucleus. 2

= recoil momentum of the nucleus. 2M M = mass of the nucleus. According to the law of conservation of momentum, recoil momentum of the nucleus = momentum of the emitted y-radiation. ER (recoil energy) =

It leads to:

E

PR

= P y =-cy,

ER

p~ =-E~- : : : -E6 =-_2. 2

C

i.e.

lic

k

H

er

e

PR , PR

2M

(c

2Mc

= -velocity of light and 2Mc

'

Ey

E

Py

= -cy)

=energy of the emitted y-photon.

=Eo-ER::::::Eo,Eo»ER • Doppler shift (En): When the emitted y-radiation from a nucleus present in a gaseous molecule moves in the same direction of the molecule, it possesses the energy which is different from the energy of the' emitted y-radiation moving iQ the opposite direction of the molecule. The Doppler effect is realised in a very common phenomenon: when a moving body emits a radiation or sound, a stationary observer experiences a shifted frequency (called Doppler shift). Here the Doppler shift arises because the radiation comes out from the moving (i.e recoiling) nucleus (cf. recoil velocity ~ 102 m S-1). However, in general, ER » ED.

(c) Energy of the y-radiation for the nuclear transition (ground level to the excited level) in the sample (i.e. absorber): It is given by:

1945

SPECTROSCOPIC METHODS AND OTHER PHYSICAL METHODS IN CHEMISTRY

E y (sample)

= ~ + ED + ER = Eo + ED + ER ::::: Eo + ER , E R »ED

For the same nuclei of the source and sample, ~E i.e. Eo is the same. ED has the same significance. When the y-radiation will be absorbed, the absorbing nucleus will also recoil. Thus the exciting y-radiation will have to provide this translational energy to the absorbing nucleus. Source (Emission)

I I I

hv

I ~--+---&..---1~

ra ry

-~

R -

(Absorption)

I I I I

Movement of the nucleus during y-emission

E

Sample

!

2Mc2

2"

yl ib

Nucleus

Line width (.1v 1)

em

(a)

(b)

al

ch

Fig. 12.4.2.1 (a) y-ray emission from a nucleus (at rest before y-emission) of mass M and imparting of recoil energy (ER ). (b) Distribution of energies of the y-radiation emitted and absorbed by the same nuclei assuming Eo (source) = Eo (sample), E R (source) = E R (sample).

C

lic

k

H

er

e

t.m

e/

th e

(d) Distribution of energies of the emitted and absorbed y-radiation: Distribution of energies of the emitted and absorbed y-rays are shown in Fig. 12.4.2.1. The breadth of the distribution curve arises from the Doppler broadening that occurs due to the t~anslational motion of the nuclei in different directions. The shaded area (i.e. overlapping region) gives the measure of the probability of the absorption of the emitted y-radiation. It is evident that the probability (measured by the shaded area in Fig. 12.4.2.1) of absorption of the emitted y-radiation is very small. It is due to the recoil energy (E R). The energy distribution curve for the emitted y-radiation is centered at Eo - ER while the energy distribution curve for the absorbed y-radiation is centred at Eo + ER , i.e. the energy mismatch factor is 2E R• This energy mismatch factor cannot be overcome by Doppler effect because E R is mach larger than ED. If the Doppler effect is to overcome the energy mismatch factor due to E R, the source .will.have to move with a velocity of 2 x 104 cm S-l which is not attainable.

Energy mismatch factor

= Ey(sample) -

E2

Ey(source)

= 2ER =~ Mc

It is evident that this energy mismatch factor is directly proportion to E~ and it is very much important in y-spectroscopy (cf optical region vs. y-ray region of the electromagnetic radiation: ca. 2 eV vs. 30

keV). If E R can be reduced, then the probability of absorption of the emitted y-radiati~=- ;l1creases. Obviously, the probability of absorption will be maximum when ER becvtnes zero (i.e. recoilless transition). E R can be reduced by increasing the mass M. This can be attained by placing the emitting nucleus (i.e. source) and the absorbing nucleus (i.e. sample) in a matrix crystal so that the effective mass (M*) becomes the mass of the crystal. Because of this effective large mass, the recoil energy (ER ) becomes less (in other words, recoil energy is readily dissipated in the matrix lattice). Thus the energy mismatch factor (2ER ) becomes less and probability of the y-ray absorption increases. Developl1ient of this technique to reduce E R was the main contribution of Mossbauf'r to develop this spectroscopic technique.

1946

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

If the source and sample are placed at low temperature, then the thermal motions of the lattice atoms (i.e. lattice vibrations) will be reduced and it will favour the recoilless transitions. This is why, MB spectroscopic studies are very oftell carried out at low temperature. (e) Natural bandwidth of the v-ray emission: Half-life of the excited Fe* nucleus is about 1.5 x 10-7 s. Using the uncertainty principle: Mx Lll=:: h, i.e. hLlV x Llt=:: h, i.e. LlV =:: ~ =

1 7 =:: 106 Hz 1.5 x 10- s Hz). It indicates that the uncertainty in energy is very small i.e. ~t

(cf associated y-ray frequency - 10 18

very small bandwidth. (~v/v ::::: 10- 12 which is much smaller than that found in other spectroscopic

ra ry

techniques e.g. LlV =:: 10-8 for NMR; LlV =:: 10-5 for IR). From Fig. 12.4.2.1, it is evident that greater v v the bandwidth, greater the probability (measured by the overlapping area) of nuclear resonance absorption.

C

lic

k

H

er

e

t.m

e/

th e

al

ch

em

yl ib

Thus the y-ray emission of very sharp frequency (i.e. narrow bandwidth) creates a problem for its reabsorption by the sample nucleus even in the condition of recoilless transition, because the different chemical environment around the sample nucleus (to absorp the y-radiation) compared to that of the source nucleus makes Eo (difference in energy between the excited and ground nuclear energy levels) of the sample slightly differentfrom the Eo of the source (to emit the y-radiation) i.e. centres of the two curves are separated by the difference of the Eo values of the source and sample nuclei even when E R = 0 (cf Fig. 12.4.2.1 where Eo is taken the same for source and sample). (f) Effect of temperature: We have already mentioned that the natural line width (determined by the uncertainty principle) of the emitted y-ray is exceedingly small (::::: 106 Hz, Vy ::::: 10 18 Hz). It minimises the possibility of overlapping between the distribution of the emitted and absorbed y-rays. At higher temperature, Doppler broadening due to the thermal motion of the emitting and absorbing nuclei in different directions is expected to increase the probability of overlapping of the two curves (Fig. 12.4.2.1) i.e. increase of the probability of MB nuclear transition in the sample nucleus. However, contrary to this expectation, it has been experimentally found that at the lower temperature, the resonance causing the MB nuclear transition becom,es favoured. It is because of the fact that at lower temperature, the reduced thermal vibration favours the recoilless transition. This aspect has been already discussed. (g) Adjustment of Doppler shift for the absorption ofy-radiation (under the condition of recoilless transition): Under the condition of E R ::::: 0, the energy mismatch for absorption of the emitted y-radiation arises for the slight difference in the Eo values of the sample and source nuclei. This energy mismatch factor ~o = Eo (source) - Eo (sample) can be adjusted by Doppler shift. If the source is moved with a higher velocity towards the sample, energy of the emitted y-radiation from the source becomes higher and vice-versa. The energy change i.e. Doppler shift (dEs) of a photon is given by: uhv uE y cose =l1Es =--cose=---c c Where ~vs = frequency shift, u = relative velocity of the source with respect to the sample, Ey = stationary energy of the photon; e = angle between the direction of the velocity of the source and the line connecting the source and sample; c = velocity of light (3 x 10 10 cm S-I). Under condition of e = o i.e. source directly moving in the direction of the sample, h~vs

uE y . UV ~s = - - I.e. ~vs = - Hz; c c

For u = 1 cm

S-I

i.e. 10-2 m

S-·I,

the frequency shift

~vs

(E y =hv) becomes about 108 Hz.

1947

SPECTROSCOPIC METHODS AND OTHER PHYSICAL METHODS IN CHEMISTRY

I8 ef. Avs '" 1 ems-I XlO 1 3xl0 0cms- 1

HZ)

r

3

4.8xl0-8 eV heVs

ch

4.8 x 1O-8 eV = 11.6 MHz 4.14xl0- 15 eVs

34

al

-

em

yl ib

AE = uEy = 1 mm S-I x14.4x10 eV =4.8xlO-8 eV s c 3xl0 11 mms- 1

ra ry

This ~vs is about 102 times greater than the natural bandwidth (== 106 Hz). In fact, varying the relative velocities from +1 cm S-1 to -1 cm S-I, a reasonable range of frequencies can be made available for scanning. Adjustment of Doppler shift to satisfy the energy condition of the y-ray absorption by the sample, generally the source is moved and the sample is kept fixed. The positive Doppler velocity or positive relative velocity means that the source is moving towards the sample while the negative Doppler velocity or negative relative velocity means that the source is moving away from the sample. Por 57Pe emitting 14.4 ke V y-ray, the energy change for the Doppler velocity of 1mm S-1 is equivalent to 4.8 x 10-8 eVe

th e

662 10-34 6.62 x 10-15) E JS= 1.6xlO- 19 eVs=4.l4xlO eVs ( C/.v=h,andh=. x

er

e

t.m

e/

Under the condition of recoilless transition, generally the energy difference (i.e. ~o) for the same nuclear transition between the sample and source nuclei is small which can be adjusted by the Doppler velocity. This magnitude and direction of the Doppler velocity give an idea regarding the chemical environment around the sample nuclei (with reference to that of the source nuclei). Obviously, if the sample and source nuclei are in the same chemical environment then for the zero Doppler velocity, the MB signal will occur provided E R = 0 i.e. recoilless transition.

H

(h) Important conclusions:

Eex

C

lic

k

----~------

Emitter

(Nuclear transition) ~ ~

(Source)

(y-absorption)

(y-emission)

_ _ _ _....z......

Absorber (sample) (Nuclear transition)

E gd

Eex

-

Egd = Eo (source)::= hv + ER (source)

E y (sample) = hv::= (Eex

2

-

E gd ) + E R

2

= E y (source) + EtL(source)

=

2M

Eo (sample) + ?R (sample) 2M

2

i.e., E y (source) = hv = Eo (source) - EtL(source) 2M

(cf. in other optical spectroscopy, Eo

=hv because E

R

is exceedingly small)

Fig. 12.4.2.2 Characteristics of the y-ray emitted and absorbed by the same nuclei.

1948

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

(i) Because of the recoil energy (E R), energy of the emitted y-radiation (Ey) is less than the energy change (Eo) during the nuclear transition from the excited state to the ground state (cf in other spectroscopic techniques, hv = f1E = Eo, where the recoil energy is negligible). For the y-photons (very high energy) emission, recoil energy is accountable (Fig. 12.4.2.1). (ii) During the excitation by absorption of y-radiation, the energy of the exciting y-radiation will have to provide the energy ofexcitation and also the recoil energy ofthe absorbing nucleus i.e.

E1 (sample) = hv = Eo + E R

= Eo +p~-

.

yl ib

broadening cannot overcome this energy mismatch factor.

ra ry

2M (iii) It indicates that for the same nuclear transition, energy of the emitted y-radiation is Eo - E R while it is Eo + E R for the y-ray absorption. Thus, the emitted radiation cannot cause the same transition and the energy mismatch factor is 2ER assuming E R (source) = E R (sample). Doppler

em

(iv) By placing the source and sample nuclei in a solid crystal (i.e. effective mass is enormously

increased), the recoil energy can be made almost zero [cf. E R

= 2MP~ =

E~ 2].

It gives the 2Meff c condition of recoilless transition i.e. for the same transition, Ey(source) : : : Eo : : : E y (sample). (iv) Due to the difference in chemical environment around the source and sample nuclei, EJ (source) :t EJ (sample). This small energy difference between EO(source) and EO(sample) can be adjusted through the Doppler shift (~v s) of frequence.

e/

= UV Hz; u = relative velocity of the source with respect to the sample; c

t.m

~v s

th e

al

ch

eff

H

er

e

(v) Avs vs. natural bandwidth: In the MB spectroscopy, natural bandwidth of the y-ray emission is extremely narrow (cf ~v/v : : : 10- 12 ) and ~vs (Doppler frequency shift:::::: 108 Hz when u = 1 cm S-I) is quite large compared to the natural bandwidth (:::::: 106 Hz). Thus, by adjusting the Doppler shift (i. e. relative velocity of the source), MB spectrum can be recorded.

C

lic

k

(i) Experimental set-up: The source nucleus (i. e. excited nucleus to be generated in situ, very short life-time) is placed in a solid matrix next to the sample matrix (sample is the same nucleus but at the ground state). A relative movement of the source will cause the appropriate Doppler shift to cause the absorption of the emitted y-radiation by the sample. A y-ray counter is placed behind the sample. The y-ray counter records the weakest signal when the sample absorbs the y-radiation. Thus a plot, signalintensity (at the y-ray counter) vs. the Doppler velocity between the source and sample gives the MB spectrum (Fig. 12.4.2.3).

y-Ray counter

velocity drive (a)

1949

SPECTROSCOPIC METHODS AND OTHER PHYSICAL METHODS IN CHEMISTRY

ca

Q)

~

5

mE (.J

c:

r.1

~ (isomer shift)

I

::s

?-

o m

0=

-2

-1 0 1 u (mm/s- )

+1

ra ry

_

yl ib

(b)

Fig. 12.4.2.3 (a) Schematic set-up of a MB-spectrometer. (b) Representation of the Mossbauer line.

em

12.4.3 Source and Absorber: MB Spectroscopy for 57Fe and 119Sn

0)

(I ~,

th e

al

ch

For the successful MB studies, life-time of the excited nucleus (to act as the source of y-emission), should lie in the range 10-6 to 10- 10 s. (a) 57Fe: The 57Pe* isotope is conveniently synthesised from 57CO = eQ < through the

1=7 2

CO

11 2

(89.5 keV)

er

e

57 27

t.m

e/

electron capture. The excited nucleus 57Fe* comes to ground state (stable 57Fe) through the y-emission (i.e. isomeric transition, IT). 57CO is prepared by electrodeposition. The complete energy diagram leading to the generation of 57Fe* which acts as the source of y-radiation in studying the MB spectrum of Fe-compounds is shown in Fig. 12.4.3.1.

H

Electron capture :::;

Y1

270 days)

k

(t 1/2

lic

(t 1/2

,

C

5 2

Y2

(100/0)

:::;

245 days)

:;Fe*(136.4 keV) Y1

(90%)

(t 1/2

:::;

I =1.

~--'-~,---_1~~Sn*

2

9

8.5 x 10- s)

(23.9 keV) YMB

'VV\N\I\/\I\I\r

(t 1/2

, YMB

(t 1/ 2

~~Fe·(14.4 keV)

1=1- - - - - - -

2

:::;

8

1.9 x 10- s) 1~~Sn

(0)

(Ground state)

7

:::;

1.5 x 10- s)

~ 18 ~

~

(y ray, _10 Hz) 57

(Ground state)

26 Fe

(0)

Fig. 12.4.3.1 Energy diagrams for the isomeric transitions giving the sources of the y-radiation (YMB) in the studies of MB spectroscopy for iron and tin compounds.

1950

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

In the transition, 57Fe*(14.4 keY) ~ 57Fe, the energy change (J1.E =Eo) per nucleus is: 14.4 keY 103 x 1.6 X 10- 19 J = 2.30 x 10- 15J. It leads to:

= 14.4 x hv

2.30 X 10-15

= J1.E = 2.30 X 10-15 J; or v =- - - h

15

2.3 X 10- J __ 3.5 x 10 18 Hz (i.e. y-radlatlon). . . 6.62 x 10-34 J s

(b) 119S0: It is prepared in the form Pd3Sn or BaSn03.' It is shown in Fig. 12.4.3.1 Some important nuclei used in MB spectroscopy are given in Table 12.4.3.1 Table 12.4.3.1 Some important Mossbauer isotopes with their precursors and nuclear characteristics. Lifetime of the first excited state (x 10-9 s)*

Gamma energy change i.e. AE (keV)

First excited state

57Fe

1/2

3/2

2.2

119Sn 1291

1/2

3/2

8.6

18.00

23.8

7/2

5/2

-0

16.30

27.8

1/2

3/2

6.7

1.40

35.6

47.8

0

2

169Tm

1/2

3/2

e/

em

14.4

ch

100

7/2

1.80

77.3

9.80

21.6

1.83

80.6

3.90

8.41

al

1/2

5/2

th e

3/2

151Eu 166Er

98.0

yl ib

Ground state

129Te 197Au

Precursor (half-life) of the y-ray source 57CO

(270 d) 119mS n (245 d) 129Te (33 d) 1251 (57 d) 197pt (18 h) 15 1Sm (90 y)

166HO (27 h) 169Er (9.4 d)

t.m

Life-time (t112) of the excited state should lie in the range: 10-6 s to 10- 10 s. If 11/2 > 10-6 s, the natural bandwidth becomes too narrow (i.e. overlapping region becomes too small, cf. Fig. 12.4.2.1) to cause the nuclear resonance absorption. If 11/2 < 10- 10 s (i.e. too short life period), the natural bandwidth becomes too broad to be useful in hyperfine studies.

H

er

e

*

Natural Abundance (%)

ra ry

Nuclear spin Nucleus

lic

k

12.4.4 Isomer Shift or Centre Shift or Chemical Isomer Shift (in short, Chemical Shift) in MB Spectroscopy

C

It has been already mentioned that even in the recoilless transition, the y-radiation emitted by the source nucleus cannot cause the nuclear excitation in the sample nucleus of the same element. It is due to the fact, that the nuclear transition energies for the sample and source nuclei are different (slightly) if their chemical environments differ. The electrostatic interaction between the nuclear charge distribution and electron charge distribution in the nucleus brings about a change in the positions ofthe nuclear energy states. This electronic charge density at the nucleus depends on the chemical environment around the nucleus. This difference (in chemical environment) between the source and sample nuclei causes the difference in the Eo values (i.e. nuclear transition energies) of the source and sample but this energy mismatch (i.e. J1.Eo = Eoource - Eoample) can be adjusted by the Doppler velocity. This energy difference t£o gives the measure of isomer shift (sometimes called centre shift, chemical shift) which is expressed in terms of Doppler velocity. It can adjust the energy mismatch to allow the nuclear transition at the sample nucleus. Thus the isomer shift (8) is expressed in em S-l or mm S-l (i.e. Doppler velocity). For [Fe(CN)6]4-, the sharp MB absorption peak arises at the Doppler velocity, ca. -0.15 mm S-I (i.e. 8 = -0.15 mm S~I) (cf. Fig. 12.4.4.2) with reference to 57Fe*, a 57Co source.

1951

SPECTROSCOPIC METHODS AND OTHER PHYSICAL METHODS IN CHEMISTRY

Due to the electrostatic interaction between the nuclear charge distribution and electronic charge distribution at the nucleus, the shift (m in the nuclear energy state is given by: dE

= KR21\11e(O)1 2

= 8(msample -

8(dE)source

.=K(R;x - R2d ) { 2 1\11 e(O)1

1 samp e

g

2}

-1\11 e(O)1 source ; K

yl ib

. 8

ra ry

where K is a nuclear constant; R = radius of the nucleus; R is different for the excited and ground state in a particular nucleus; l\IIe(O)1 2 = total electron density at the nucleus; Thus the total energy change in a particular nuclear transition due to the above interaction is given by: 8(m = dEex - dEgd = K1\11e(O)1 2 (R;x- Rid) Thus the isomer shift (8) which gives the measure of the difference in the nuclear electrostatic energy changes of the source (i.e. y-ray emitter) and sample (i.e. y-ray absorber) is given by:

2 2 = -1tZe 5

em

It is obvious that if the sample (i.e. absorber) and source (i.e. emitter) are the same nucleus with the same chemical environment then ~ becomes zero.

ch

al

(ex) -----r----------------------------

th e

t

~---r-----~r--- (ex),1 = ~ 2 ~Eex

H

er

e

t.m

e/

I=

/ //

2

lic

k

(a) l'Vs(O)1 = x (say)

(b) l'Vs(O)1

2

= y (say)

C

Fig. 12.4.4.1 Schematic representation of the energy change in MB transition for 57Fe depending on the values of I'll/0)1 2 i.e. electron density at the nucleus (M 1 ) 11£2 i.e. higher electron density at the nucleus reduces the MB transition energy).

For a particular element, a fixed and standard source is used, e.g. 57CO for Fe; Ba119mSn0 3 for Sn; etc. Thus, l\IIe(O)I~ource may be taken as a constant, P (say). When (Rex - Rgd ) is very small, we can reasonably write:

It gives:

1952

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

i.e.

B=K,(BR)[I R 'Ve (0)1

2

-P].,K'=!1tZe R 5 2

sample

2

where K' = 2KR 2 (a constant and positive); oR or dR is also a constant for a given nucleus.

R

R

Now let us consider the electron density at the nucleus. For the different orbitals like s, p, d, f, except the s orbital, all other orbitals have the node at the nucleus (cf. Figs. 4.1.4.1-2, Vol. 1). Thus only the s-electron can interact directly with the nuclear charge. However, other orbital electrons can indirectly control the s-electron density through their shielding properties. Thus we can write: 000'

and 0 =

~

u

(Rex < Rgd)

·S

()

-5

~

cI

~

0

0

~

~

(,)

00

~

'So

"'0

clj

11)

dR or8R = +ve

al

"'0 C

C

11)

11)

[Fe(CN)6t(S7 Fe)

"E

C

en

em

U

11)

Q)

Chemical term = +ve Co

ch

11)

dR or 8R = -ve

en Co 0en

..c u

~

()

Q)

11)

czi

K'( 0; )[1'" s(o)l:amPle - P]

ra ry

2

yl ib

I", e(0)!2 ." I", ,(0)1

a clj

()

Q)

en Co 0en

"E ~ o

(Rex) Rgd) dR or 8R = +ve

(,)

00

11)

-5

Chemical term = -ve

CoM

for both

0

1271 and 129 1

11) ~

£3 00

• •

.--.....

Relative velocity (mm/s)

8(129 1)

(c)

Fig. 12.4.4.2 (a) Negative isomer shift for [Fe(CN)6]4-; (b) Positive isomer shift for Sn(C 2H s)4; (c) Positive isomer shift for Na3H2127I06 (Source: Zn l27 Te i.e. zinc telluride containing l27mTe) and negative isomer shift for Na3H2129I06 (Source: Zn 129mTe).

1953

SPECTROSCOPIC METHODS AND OTHER PHYSICAL METHODS IN CHEMISTRY

Nuclear term and chemical term: The isomer shift 8 depends on the nuclear term K'(8R1R) and chemical term

[Iv s(O)1

2 samp1e -

p] i.e. [Iv s(o)1

2

samp1e

-Iv s(O)1

2 source ].

Thus the chemical term indicates

H

er

e

t.m

e/

th e

al

ch

em

yl ib

ra ry

the difference in the electron densities at the sample (i.e. absorber) and source (i.e. emitter) nuclei. The nuclear term is a constant for a particular Mossbauer transition and consequently, isomer shift depends only on the chemical term. This is why, the isomer shift is called the chemical isomer shift. • If 8R is positive and chemical term is positive, i.e. l'Vs(O)I~arnple) l'Vs(O)I~ource' then 8 (isomer shift) is positive; but for the negative value of 8R and positive value of chemical term or vice versa, 8 becomes negative. (A) Sign of 8R (= Rex - R gd ), sign of chemical term and sign of isomer shift: Sign of 8R depends on the relative magnitudes of Rex and Rgd • For 57Fe, Rex ( R gd i.e. SR is negative and increase in s-electron density in the sample will produce the isomer shift more negative (cf. Fig. 12.4.7.1, Table 12.4.7.1). For 119Sn, Rex ) R gd i.e. 8R is positive and increase in s-electron density gives a more positive chemical shift. Sign of the chemical term depends on the relative magnitudes of l'lfs(O)I~arnpleand l'Vs(O)I~ource i.e. it becomes positive for IWs(O)I~ample) IwS Rgd ). For the Sn(IV) (5so 5po) compounds, the isomer shift is about

1974

FUNDAMENTAL CONCERtS OF INORGANIC CHEMISTRY

2.7 to 3.5 mm S-1 less positive than that found for the Sn(ll) (5s 2 5p O) compounds. The magnitude of isomer shift depends on the electronnegativity of the groups or atoms attached with the metal centre. Higher electronegativity of the group withdraws the more electron density from the metal centre to decrease the 8-value more (ef. less the s-electron density, less the posi~ve 8-value, i.e. shifting of 8 towards the negative direction). Empirically, the following relations have been established. [SnX 4 ](sp3_Sn): 8 (mm S-I) = 4.82 - 1.27Xp (X) [SnX 6 ]2-(sp 3£i2-Sn): 8 (mm S-I) = 4.27 - 1. 16Xp (X)

ch

em

yl ib

ra ry

• 0 ([SnX4]) ) 0 ([SnX6]2-): Addition of two extra ligands (i.e. X") to SnX4 is expected to increase the electron density around Sn to enhance the isomer shift. But the experimental findings do not support this prediction. It can be rationalised by considering the bond lengthening effect (ef Sn-X bond is longer in [SnX 6]2- than that in [SnX4]) and the change of hybridisation (ef. approximately Sp3, i.e. 25% s-character, in SnX4, and approximately sp3£i2 i.e. 16.6% s-character, or the multicentred bonding by using the p-orbitals in [SnX 6]2-, ef Chapter 9, Vol. 2). The bond lengthening in [SnX 6]2- due to the increased steric crowding and average less s-orbital character of Sn in the Sn-X bond of [SnX 6]2- leads to the less electron receiving by Sn in its Ss-orbital from the ligands in [SnX 6]2(compared to SnX4). This explains the observed isomer shifts. The relative increase in the p-character of the hybrid orbitals of Sn in [SnX 6]2- enhances the electron density in the p-orbitals of Sn through the receiving electron from the ligf'.nds (i.e. Lewis bases).

th e

al

This causes more screening of the 5s electrons by the increased p-electron density of Sn to decrease the isomer shift in [SnX6]2-. This also explains the isomer shift order.

k

H

er

e

t.m

e/

• 0 (trans-[R 2SnX4 ] ( 0 [R2SnX2]: This order can be explained in the same way as in the case of [SnX4] vs. [SnX6]2- (i.e. sp3 vs. sp3J2). Here it is to be mentioned that the 5s electron density of Sn is mainly concentrated along the Sn-C bonds. Use of the sp3£i2 hybrid orbitals (16.6% s-character) indicates the relatively less electron receiving in the 5s-orbital of Sn from the ligands compared to the compounds where Sn uses the sp3-hybrid orbitals (25% s-character). This less electron receiving in the Ss-orbitals of Sn in [R2SnX4] makes 0 relatively less. Use of more p-character and d-character in the hybrid orbitals (projected towards the Sn-X bonds in trans-[R 2SnX4 ] , cf Bent's rule) of Sn in [R2SnX4] causes an enhanced electron density in the p- and d- orbitals and this causes more screening of the Ss electrons. It makes also the isomer shift (0) less for trans-[R 2 SnX4 ].

C

lic

• Relative isomer shifts in [SnBr4X2]2- (X = F, CI, Br, I): In these 6-coordinate complexes of Sn(IV), the metal centre (Sn IV ) receives electron from the ligands which act as the Lewis bases. Obvously, Sn(IV) receives less electron in its valence orbitals (4d, 5s and 5p; probably sp3J2 hybrid orbitals) from the more electronegative ligands. The effect of electronegativity of the groups attached to Sn on the isomer shift (8) is illustrated in the following examples:

0 (mm

Complex

(Et4N)2[SnBr4P2] (Et4N)2[SnBr4CI2] (Et4N)2[SnBr6] (Et4N)2[SnBr412]

~

0.77

b....

0.84

Or;)

~ ~

~

'XP (X)

0.53

on

c:

S-l)

0.96

The complex anion, [SnBr4X2]2- is supposed to bear the sp3J2 hybridised Sn(IV) i.e. all these anions where X- = P-, CI-, Br-, 1- are structurally similar. In moving from X- = P- to X- = 1-, the electron accumulation at the metal centre gradually increases (ef electronegativity order: P ) Cl ) Br ) 1). Thus the electron density and consequently the s-electron density (which determines the magnitude of 8) is

1975

SPECTROSCOPIC METHODS AND OTHER PHYSICAL METHODS IN CHEMISTRY

maximum in the iodido complex while it is minimum in the fluorido complex. It explains the order of

8 i.e. highest value for the iodido complex and the lowest value for the fluorido complex.

(mm

Compound

S-l)

-0.47 -0.49 0.85 1.15 1.55 1.20 1.22

SnI4 SnMe4 SnPh4

em

SnI 2 Sn(CsHsh

~

(mm

S-l)

3.15 3.90 3.20 4.05 3.95 3.85 3.75

e/

Isomer shifts relative to Sn02'

SnF2 (orthorhombic) SnCl 2 SnBr2

ch

SnBr4

SnS04

al

SnC1 4

SnS

th e

SnF4 Na2[SnF6]

yl ib

~

Compound

ra ry

Note: No equadrupole splitting is found in the MB spectrum of [SnBr2CI4]2- and [SnBr4CI2]2-. It indicates that the unsymmetrical ligand substitution in the present octahedral species is not sufficiently high to cause the electric field gradient (EFG) big enough to cause any detectable quadrupole splitting. • Sn(II) vs. Sn(IV) compounds: In Sn2+ (5s 2 ), the 5s-electron density at the nucleus is more than that in the Sn-atom (5s 2 5p2) because the screening effect of the p-electrons in Sn 2+ is absent. In Sn4+ (5so), the 5s-electrons have been removed and the s-electron density at the nucleus is reduced. In general, for 119Sn (8R/R is positive), the isomer shift of the Sn(II) compounds becomes greater than 2 mm S-l while it lies below 2 mm S-l for the Sn(IV) compounds (both relative to Sn02)' This may qualitatively help to identify the oxidation states of So.

H

er

e

t.m

The isomer shift of (SnPh2)n (polymeric form) is 1.55 mm S-l which indicates the oxidation state of Sn as +4 not +2 (though apparently indicated so). Thus comparison of the isomer shift helps to identify the oxidation state of tin. • Order of isomer shifts in R 2SnX2 and R 3SnX (X = F, CI, Br, I): It follows the order: 8 (I) ) 8 (Br) ) 8 (CI) ) 8 (F) i.e. with the increase of electronegativity of X, the isomer shift decreases as expected.

C

lic

k

Note: If the electronegativity of X can direct Sn to change its state of hybridisation, then the prediction of the direction of Dfrom the knowledge of electron depletion by the electronegativity effect of X will not be correct. It will be illustrated below, by taking the mixed ligand organotin compounds). • 8: [R4Sn] ( [R3SnX] ( [R2SnX2] (X = F, CI, Dr, I): With the increase of the number of the electronegative substituent X, the isomer shift increases. It is illustrated:

8 (mm

S-l):

[Me4Sn] 1.20

[Me3SnCI] 1.47

[Me2SnCI2] 1.56

If we consider the additive effect of the electronegativity of X, then with the increase of he number of X, there will be a more electron depletion from the Sn-centre to decrease the isomer shift more. How0ver, the experimental findings do not support this prediction from the additive effect of electronegativity. To explain the experimental finding, it is required to consider the inequivalence of the ligands (specially in the mixed ligand organotin complexes) to change the mode of hybridisation at the Sn-centre. According to Bent's rule (cf Chapters 9, 8; Vol. 2), the more-electronegative groups (i.e. X) will try to use the more p-orbital enriched (i.e. less s-orbital enriched) hybrid orbitals of Sn. Consequently, the

1976

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

t.m

e/

th e

al

ch

em

yl ib

ra ry

more s-orbital enriched hybrid orbitals of Sn will be projected towards the less electronegative R groups. It is indicated by the increase of C-Sn-C bond angle (from 109° ~ 180°) with the increase of the additive electronegativity of X, i. e. with the increase of the number of X and increase of electronegativity of X, the C-Sn-C bond angle opens up. This opening of the C-Sn-C bond angle indicates the localisation of the 5s-electron on Sn along the Sn-C bonds. This explains the trend of isomer shift: [R4Sn] ( [R3SnX] ( [R2SnX2] (cf C-Sn-C bond angle sequence: [R4Sn] ( [R3SnX] ( [R2SnX 2]) • cis-[R2SnX4l (8 = 0.7 - 1.0 mm/s) vs. trans-[R2SnX 4 l (8 = 1.2 - 1.6 mm/s): According to the Bent's rule, the more s-character enriched hybrid orbitals of Sn are projected towards the less electronegative R-groups while the more p-character enriched hybrid orbitals of Sn are projected towards the more electronegative X-groups. It leads to: localisation of the 5s-electrons on Sn along the Sn-R bonds and receiving of electrons by Sn in its p-character enriched hybrid orbitals from the X-groups (i.e. ligands) (cf increase in the p-electron density screens the s-electrons more). Now let us compare the said cis- and trans- isomers. In the trans-compound, the C-Sn-C bond angle is close to 180° (i.e. 5s-electrons of Sn more concentrated on Sn along the Sn-C bonds) while the corresponding bond angle in the cisisomer is far less than 1800 • In the trans-compound (C-Sn-C bond angle -- 180°), approximately the sp-hybrid orbitals of Sn are involved to make the two trans- Sn-C bonds, i.e. the 5s electron of Sn gets almost completely localised along the two trans-axial Sn-C bonds. On the other hand, for the cis-compounds (C-Sn-C bond angle « 180°), the hybrid orbitals making the Sn-C bonds bear relatively less 5s-orbitals of Sn i.e. the 5s-electron of Sn in the cis-compound is less localised. This inequivalence of the hybrid orbitals of Sn towards the different ligands leading to the localisation of 5s-electron density of Sn to different degrees in the cis- and transisomers can explain the order of isomer shift.

er

e

Sn-Compounds: Additive electronegativity effect vs. inequivalence of the hybrid orbitals in controlling the isomer shift.

C

lic

k

H

(a) It has been evident from the above discussion that for a giv~n coordination number, if all the ligands are similar in electronegativity or hardness and mode of hybridisation of Sn remains the same for the series of compounds, then the effect of additive electronegativity will determine the order of isomer shift as in the cases of (i) [SnX4]; (ii) [SnX6]2-; (iii) [R2SnX 2]; (iv) [R 3SnX]; (v) [SnBr4X2]2-, etc. for X = F, CI, Br, I (b) In the mixed ligand organotin compounds, as for the series (i) [R4Sn], [R3SnX] and [R2SnX2]; (ii) cis- and trans- isomers of [R 2SnX4], the inequivalence of the hybrid orbitals towards the different ligands becomes the important factor to control the isomer shift. In such cases, localisation of the 5s electron on Sn will increase the isomer shift and the demand of additive electronegativity may be denied. 16. 12CI 6 vs. I2Br2CI4 (Ref. M. Pasternak et aI, J. Chem. Phys. 48, 1997, 1968): The covalently bonded 12Cl6 possess a planar bridge structure where both the I-centres are equivalent. For 1291, the ground and excited states are characterised by I

5 3 25 (.I.e., m/ = ±2' ±2' ±21)

= '2 (i.e. 2

m/ = ±2,2 ±~,2 ±~,2 ± -.!..) 2

and

respectively. Thus the noncubic chemical environment gives a complex

pattern of quadrupole splitting into 8 components for the transition I =

'2 ~ I = ~ 2

2

maintaining

1977

SPECTROSCOPIC METHODS AND OTHER PHYSICAL METHODS IN CHEMISTRY

CI~/B~/CI

/I~/I~ CI

Br

CI

em

yl ib

ra ry

(b) D2h

• Two set of lines ~ I-atoms nonequivalent. • 8 (I a ) ~ 3.5 mm/s; 8 (I b) ~ 2.8 mm/s (structure d, Fig. 12.4.7.5a)

e/

th e

al

• One set of lines ~ both the I-atoms equivalent • 8 ~ 3.5 mm/s

ch

Fig. 12.4.7.5 (a) Structures of I2Cl 6 and some possible structures of I2Br2C14'

t.m

r u Q)

e

en

er

Q5

0-

en

H

'E ::::J

o

Q Q

C

lic

k

()

Doppler velocity

Fig. 12.4.7.5 (b) Comparison of the partial 129I_MB spectrum of I2Cl6 and 12Br2C14' Note: 3 quadrupole splitting components (P, Q, R) out of the 8 quadrupole components are shown for I 2C1 6 • Both the I-centres being equivalent shows only one signal which is split into 8 components (P, Q, R, ... ) due to the quadrupole interaction. In I 2Br2C1 4 , two I-centres being nonequivalent show two MB signals and each signal is split into 8 components (P, Q, R, ... ; p', Q', R', ... i.e. P, Q, R, ....... represent one I-centre identical to the I-centre of 12Cl 6 and p', Q', R' ...... represent the other I-centre). R' remains hidden under the shoulder of Q.

= 0, ±1 (ef Fig. 12.4.5.1). For 12Br2C1 4, different possible bridge structures may be proposed (Fig. 12.4.7.5a) but the correct one may be identified by the MB spectroscopy for 1291. MB spectroscopy results indicate that in 12Br2C14, the two iodine centres are in different environments (i.e. two different sets of signals having appreciably different isomer shift values) and one I-centre in 12Br2CI4 is identical to the I-centre present in 12C1 6 ~ml

1978

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

t.m

e/

th e

al

ch

em

yl ib

ra ry

In the possible structures (b) and (c) of I2Br2C14, the I-centres are equivalent and they cannot explain the MB spectrum of 12Br2C14. The structure (d) shows the presence of two iodine centres in different environments and it is in conformity with the observed MB spectrum (Fig. 12.4.7.5b). Here it may be noted that in the structure (d) of 12Br2C14, one iodine centre (Ia ) is approximately identical to the iodine of 12C1 6 • 17. IF6+vs. IF6-: The MB spectrum of 1291for (IF6)+(AsF6)- shows a single line without any quadrupole splitting. It indicates the octahedral symmetry of the cation 1Ft. The MB spectrum of 1291 for IF6- in CsIF6 (supposed to be Cs+ IF6-) shows the quadrupole splitting (expected into 8 components) which is almost symmetrical. It indicates the noncubic symmetry in IF6-. IF6- actually represents a pentagonal bipyramidal structure where one basal position is occupied by a stereochemically active lone pair. This noncubic symmetry causes the quadrupole splitting which indirectly supports the presence of a stereochemically active lone pair. If the lone pair were stereochemically inactive (to be housed in the inner s-orbital which is spherically symmetric) then IF6- would have adopted the cubic symmetry to deny the quadrupole splitting of the MB signal. • TeF~- vs. IF6" (stereochemically inactive lone pair vs. stereochemically active long pair): 129Te-MB spectrum of TeX 62- indicates no quadrupole splitting. It supports the perfect octahedral structure of TeX~- Le. the stereochemically inactive lone pair is housed in the inner 5s orbital. But the quadruple splitting in the 1291_MB spectrum of IF6- (isoelectronic with XeF6) indicates the presence of the stereochemically active lone pair, i.e. structure of IF6- is not octahedral.

IF;(EFG = 0) No quadrupole splitting

e

c.>

Q)

er

en Q5

a.

H

en

C ::J

k

o

C

lic

()

IF~ (EFG ~ 0) (Quadrupole splitting)

F Stereochemically active lone pair

F- - - - - - - - - - - --

Doppler velocity (a)

//~ F\ , ,

,, ,,

,

i

F ....

.... ........

I

~

........ F I -------........

...

........

........

(b)

Fig. 12.4.7.6 (a) Partial (noncubic symmetry).

129

1 MB spectra ofIF6+ and IF6-; (b) structures ofIF6+ (octahedral i.e. cubic symmetry) and IF6-

1979

SPECTROSCOPIC METHODS AND OTHER PHYSICAL METHODS IN CHEMISTRY

18. Structure of 13-: MB spectroscopic study of 1291 in the compounds like Cs1 3, indicates the presence of two types of l-centres, i. e. two different signals having the intensity ratio 2: 1. Thus any structure proposing the equivalent I-centres is not acceptable. The linear structure of 13- is in conformity with the MB studies and it is also supported by the crystal structure. 19. XeCI4 and XeF4 : The ~-decay product of K[129ICI4]·H20 is XeCI4. Comparison with the 129Xe-MB spectrum of XeF4 suggests XeC14 to be planar. -~-) 129 XeCI ,4

129 1C I4

(planar)·'129 104

-~-) 129 XeO (tetrahedral) 4

Quadrupole splitting is expected for the planar geometry but not for the tetrahedral geometry.

ch

em

yl ib

ra ry

Both XeF4 and XeCl4 (obtained from 129ICI4-) show the characteristic quadrupole-split doublet (tillQ = 41 mmls for XeF4 and 26 mmls for XeCI4) with the characteristic isomer shift ea. 0 mmls (Fig. 12.4.7.7). Xe04 obtained from 129104- shows no quadrupole splitting in the 129Xe-MB spectrum because Xe04 is tetrahedral (EFG = 0). 20. SnX4 : With the increase of electronegativity of X, there will be more electron depletion from the s-orbital of Sn (assuming the Sp3 hybridisation) and consequently, the isomer shift (for the MB spectrum of 119Sn) becomes more negative (i.e. less positive) with the increase of electronegativity of X (ef dR is positive for, i. e. Rex:> Rgd ).

al

(Tetrahedral)

Xe _MB spectrum (XeCI 4 )

th e

I

/7/

t.m

EFG=O

~x~ ~~ quadrupole

e/

129

L

L

L

er

e

L

~litting

() Q)

C o

,:::J

Xe _MB spectrum (XeF4 )

L

~

~

C

lic

k

o

i

129

H

~ (J)

-40

L/ -20

0

+20

L

/(Planar)

EFG=t=O xe"" Qu~~rupole " " splitting

L

+40

Doppler velocity (mm/s)

Fig. 12.4.7.7 129Xe-MB spectra ofXeF4 and XeCl4 (obtained from the r3-decay of 12YIC14-). Electric field gradient (EFG) = 0 for the tetrahedral geometry (i.e. no quadrupole splitting), =I:- 0 for the square planar geometry (i.e. quadrupole splitting).

21. SnF4 vs. SnCI4 : The MB spectra of 119Sn indicates the quadrupole splitting for SnF4 but no such quadrupole splitting is noticed for SnCI4 • In fact, SnCl4 exists as a tetrahedral monomeric species (m.p. -33°C) while SnF4 exists as a 2 D-Iayer polymeric species (high m.p.) where the octahedral coordination around Sn is maintained by sharing the 4 equatorial apexes of the octahedral 'SnF6' units. Thus, in the polymeric structure of SnF4, the octahedral 'SnF6 ' units are joined into the planar layers by corner sharing of the four equatorial F-atoms (which act as the bridging ligands). The

trans-axial Sn-F bonds (terminal Sn-F bonds, 188 pm) are shorter than the equatorial bridging Sn-F bonds (202 pm).

1980

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

188 pm

CI

\.~ / 202pm~I / F

i

F

/

I/F

-F-M-F-M-F_

F-

/1

/1

~F F~~/F F~

/1 /F

F

F

/1

/F

F

~

I I "'CI

Sn

CI/

CI (Tetrahedral)

L/12-F

=

(SnF )x

(MF4)x (M Sn): 2D-layer structure (Sharing of 4 equatorial apexes of MF6 octahedra)

ra ry

4

Thus, in the polymeric structure of SnF4 , there are two types of Sn-F bonds and it gives an asymmetric distribution of electron cloud around the Sn-nucleus and it causes the quadrupole splitting

H

er

e

t.m

e/

th e

al

ch

em

yl ib

of the MB spectral signa~ of lI9Sn. On the other hand, in SnCl4 (monomeric form), the tetrahedral (i.e. cubic ligand field symmetry) structure maintains the spherical distribution of the electron cloud around the Sn nucleus and consequently, the MB spectral signal does not experience the quadrupole splitting. 22. (a) [SnCI4] vs. [SnR4]; (b) [SnR4], [SnR3X], [SnR2X2]; (c) [SnX4] vs. [SnX6]2-; (d) [SnR2X4] (cis- vs. trans isomers); (e) SnR2X2 (X = F, CI, Br, I): The relative values of isomer shift for such compounds of tin have been discussed and explained earlier in this section. 23. Biological system: Iron-proteins (heme-proteins, non-heme proteins, Fe-S protein etc.) have been widely subjected to the MB spectroscopic study to identify the oxidation states. Some examples are given below: • Deoxy- and oxy-hemerythrin: The MB spectrum clearly indicates the two types of Fe-centres in the oxy-Hr form but MB spectrum cannot distinguish the Fe-centres in the deoxy-Hr form. It is reasonable because in the deoxy form, the two Fe-centres are in the slightly different environments (basically in terms of the amino acids) but the Fe-centres are in the widely different environments in the oxy-form where one Fe-centre binds with the peroxide moiety while the other centre remains almost in the same environment as in the deoxy form.

Deoxy-Hr

Oxy-Hr

lic

k

Mossbauer data and Magnetic properties

C

Isomer shift (8 in mmls): 1.14 ] 0.52] 0.48] Quadrupole splitting One doublet Two doublets (dEQ, mmls): 2.76 1.92 1.00 J (cm- I ): .... -10 .... -80 Here it is worth mentioning that in deoxy-Hr, the ~-hydroxido moiety (Jl-OH) brings about a weaker antiferromagnetic coupling interaction (J::::: -10 cm- I ) while in the met-Hr or oxy-Hr having the J..I.-oxido grdup (i.e. Jl-O), the antiferromagnetic coupling interaction is stronger (J ::::: -80 em-I).

In fact, compared to the J..I.-OH group, the J..I.-O group is more efficient in bringing about the antiferromagnetic coupling (see Chapter 8). • Oxidation state of iron in the active form of catalase and peroxidase: The enzyme in its resting condition represents the Fe(III)-heme protein, which on being treated with H20 2 produces two compounds described as compound-I (green, Amax = 405, 655 nm) and compound-II (red, Amax = 429, 536, 568 nm). The compound-I is more reactive than the compound-II. Fe III _ P ~ Red compound~ Green Compound (p for porphyrin)

(Compound II)

(Compound-I)

1981

SPECTROSCOPIC METHODS AND OTHER PHYSICAL METHODS IN CHEMISTRY

H

(H~S) (His)N

!i'O ' 0

I /6~

~FIIIe

(HiS)N/

/N(HiS)

e III

I ""0/ I~

O~ON(HiS)

J,!-hydroxidodiiron(lI) system, weak

J,!-oxidodiiron(llI) system, strong antiferromagnetic couping (J ~ -80 cm -1) diamagnetic; high spin Fe(III).

1

ra ry

antiferromagnetic coupling (J ~ -10 cmparamagnetic; high spin Fe(II).

),

yl ib

Fig. 12.4.7.8 Structures of the deoxy- and oxy-hemerythrin.

em

(See the author's book, Bioinorganic Chemistry for details).

(0 = O.S2mm/s)

naphthalide in THF)

er

e

t.m

e/

th e

(0 = O.45mm/s)

+_le_ _~) (Reduction by sodium

al

FeIII(TPP) ~ FeII(TPP)

ch

Isomer Shift in Iron-Porphyrin Systems: Identification of the Oxidation State of Fe [Na(THF)3J[Fe(TPP)] (0=O.65mm/s)' (Fe!, S = ~).IOW spin

t~:duction

1

by sodium naphthalide)

[Fe (TPP)J2- (diamagnetic)

( = 0.48 mm/s)** (Fell) (not Fe 0 )

k

The isomer shift (0.65 mmls) indicates that the added electron is taken by Fe(ll) to give the

lic

*

H

FeII(TPP) i.e. Fe(TPP) is the tetraphenylporphyrinate complex of Fe(II).

**

C

low-spin Fe(l) ( S =

-i)

(cf. Fig. 12.4.7.1;

8 '" 2.2 to 2.3 mm/s for high spin Fe(I».

The isomer shift (0.48 mmls) indicates that the t'wo electrons added to Fell(TPP) are probably taken by the porphyrin ligand not by the Fell-centre. However the single electron reduction occurs at the metal centre. • Isomer shift of Fe IV -haemproteins: - 0 mmls

Compound-II is a compound of FeIV as FeIV(p)(O) i.e. oxido-complex of Fe(IV). But the complex-I may be described either as a compound of Fe(V) like Fev(p)(O) i.e. oxido-complex of Fe(V) or as a compound of Fe(IV) like FeIV(p+)(O) i.e. oxido-porphyrin radical complex of Fe(IV). The MB spectra of the compound-land compound-II can be best interpreted in terms of the Fe(IV) oxidation state (Fe ~ 0.05mm/s, dE Q ~ 1.6 mm/s). These MB-parameters nicely agree with those of the authentic FeIV-porphyrin complexes (Fig. 12.4.7.9). Thus the green compound i.e. compound-I is a ferryl complex i.e. Fe(IV)-complex not Fe(V)-complex and in generation of the green complex from the red complex, the electron is lost from the porphyrin moiety not from the metal centre.

1982

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

1v

Fe -porphyrin complex

(EFG

* 0)

8 ~ 0 mm/s (cf.Fig.12.4.7.1) ~EQ ~ 2 mm/s

-2

--+~I

o

-1

+2

+1

+3

em

Doppler velocity (mm/s)

yl ib

-3

ra ry

I I

·+-

ch

Fig. 12.4.7.9 Qualitative representation of the zero-field MB-spectrum of a representative FeIV -prophyrin complex.

%) while the reduced form bears FelI(tfi, S = 2). In the other

th e

and the oxidised form bears FeIIl ( d 5 ,S =

al

• Fe-S proteins (ferredoxins denoted by Fd, in general): The simplest Fe-S protein is rubredoxin (Rb) where the iron centre is tetrahedrally surrounded by four cysteine-S sites. The structure is distorted

e/

Fe-S proteins e.g. Fe2Sb Fe3S4, Fe4S4 etc. there are more than one Fe-centre and these remain coupled

t.m

(see the author's book, Bioinorganic Chemistry for details). In these multinuclear Fe-S proteins, distorted tetrahedral geometries having the mixed ligands (i.e. inorganic sulfur and cysteine-S). The MB spectral studies of the Fe-S proteins have been found quite helpful in understanding the structure and activity of the different Fe-S proteins. The MB spectra of some Fe-S proteins are given in Figs. 12.4.7.10-11. It is expected that in general, in the Fe-S proteins, the quadrupole splitting of Fe 3+ (having the symmetric distribution ofd-electrons i.e. e2 t1) will arise only from the distorted tetrahedral structure (slight distortion; measure of deviation from the cubic ligand field symmetry) and inequivalence of

C

lic

k

H

er

e

Fe 3+(d5 ; e2ti) and Fe 2+(tf : e3ti) reside also in the

s/ (Rb)

I

Cys 'VVVV'v-

[Fe(S-Cys)J

Fe

r/sl ~S-CYS ~

Doublet for Fe(lI) -./VVVV'

-4

-2

o

+2

Cys

"

Cys'VVVV'v-

+4

Doppler velocity (mm/s) (a)

(Contd... )

1983

SPECTROSCOPIC METHODS AND OTHER PHYSICAL METHODS IN CHEMISTRY

fYS~ 8

IYS~ 8

8

"'-/"'-/ Fe

+- Doublet for Fe(lII)

8/ '

o

Q)

/Fe

"-8/

I

~ E ::J

Cys'VVVVV-

o

" 8

I

Cys'VVVVV-

()

0

2

4

ra ry

-2

-4

Doppler velocity (mm S-1)

em

yl ib

(b)

ch

Doublet for Fe(lII)

~ "E

al

(,) Q)

-4

e/ o

-2

t.m

o

th e

Outer doublet for the delocalised Fel"-Fe" (coupled unit) Le. Fe2.5+

:J

()

+4

+2 1 )

er

e

Doppler velocity (mm 5(c)

lic

k

H

Fig. 12.4.7.10 Qualitative representation of the MB spectra of some representative Fe-S proteins. (a) Oxidised and reduced forms of rubredoxin (lFe-OS i.e. no inorganic sulfur) denoted by Rb; Rb (Felli) and Rb d (Fell). (b) Oxidised and reduced forms of 2Fe-2S ferredoxin denoted by Fe2S2-Fd; Fe 2S2-Fdox (2Fe IIl ) and Fe 2S;-Fdred (Felli + FeU). (c) Oxidised and reduced forms vf 3Fe-4S ferredoxin denoted by Fe 3S4-Fd; Fe 3S4-Fd ox (3Fe llI ) and Fe3S4-Fdred (FellI + coupled FellI-Fell unit not 2FellI + IFell ).

C

the ligands (i.e. inorganic sulfur and cysteine sulfur). On the other hand, for Fe2+ (having the asymmetric d-electron distribution, e 3tl), the quadrupole splitting arises from the asymmetric d-electron distribution itself, distortion (J.T. effect) and ligand nonequivalence. Thus it is quite reasonable that the magnitude of quadrupole splitting will be more for Fe(II) i.e. ~EQ (Fell) » ~EQ (FellI) in Fe-S proteins. This prediction has been supported experimentally. (i) For rubredoxin (Rb), [Fe(SR)4]' in the oxidised form, the MB spectrum of 57Fe shows a small quadrupole splitting (ca. 0.7 - 0.8 mm/s) with the isomer shift 0.25 mm S-1 while the MB spectrum at the reduced state shows the higher magnitude of the quadrupole splitting (ca. 3 - 3.5 mm/s) with the isomer shift 0.65 mm/s. It indicates that in the oxidised form of Rb (i.e. Rb ox ), Fe(III) is present while in its reduced form (i.e. Rb red ), Fe(II) is present. (ii) For Fe2S2 ferredoxin, the 57Fe-MB spectrum of the ,oxidised form shows a single doublet with a small quadrupole splitting and isomer shift 0.26 mm/s (cf. 8 = 0.25 mm/s for the oxidised form of Rb), a characteristic feature of Fe(III). It indicates that in its oxidised from, both the Fe-centres are as the Fe(III) centres in equivalent positions. In the reduced form, the MB spectrum shows

1984

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

a smaller inner doublet (i. e. smaller quadrupole splitting, characterising the Felli-centre) with the isomer shift 0.25 mmls and a larger outer doublet (i.e. higher quadrupole splitting, characterising the Fell-centre) with the isomer shift 0.55 mmls. The peak area is in the 1: 1 ratio. The isomer shift 0.55 mmls with a higher quadrupole splitting supports the +2 oxidation state of iron (el 8 = 0.65 mmls for Fell of rubredoxin). Thus the MB spectroscopy proves that in the oxidised form of Fe2S2(Fd), both the Fe-centres are as the equivalent FellI-centres which are antiferromagnetically coupled (i.e. S 0) while in the reduced form it contains localised one FellI-centre and one Fell centre; and these localised FellI and

=

Fell centres remain antiferromagnetically coupled (i.e. S = !). 2

2Fe-2S Fd (i.e. Fe2SrFd)

em

0.25 (oxidised form, FellI) { 0.26 (oxidised form, 2FeIH ) 0.65 (reduced form, Fell) 0.25 and 0.55 (Reduced form, { Fe JII + Fell)

ch

8(mmls):

Rubredoxin (Rb) (lFe-OS)

yl ib

Fe-S protein:

ra ry

Table 12.4.7.3 Isomer shifts (8) of some Fe-S proteins.

{

3Fe-4S Fd (i.e. Fe3S4-Fd)

0.27 (oxidised form, 3FellI ) 0.29 and 0.45 (Reduced form, 2Fe llI + Fell) Probably IFe3+ + 2Fe2.S+

C

lic

k

H

er

e

t.m

e/

th e

al

(iii) For Fe3S4 (denoted by Fd-II having the void cubane structure), the 57Fe-MB spectrum of the oxidised form shows one small doublet (i.e. smaller quadrupole splitting, ~EQ = 0.53 mmls) with the isomer shift 0.27 mmls and it supports the presence of three equivalent FellI-centres. On the other hand, the reduced form (produced by the addition of one electron to the oxidised form constituted by 3Felll -centres) shows an outer doublet (larger quadrupole splitting) with 0.45 mmls isomer shift and an inner doublet (i.e. smaller quadrupole splitting) with the isomer shift 0.29 mmls. If the reduced form is assumed to be constituted by 2Fe lll and 1Fell centre, then inner doublet should correspond to the FellI centres and the outer doublet should correspond to the Fell centre. But this assignment raises two questions: • 8 = 0.45 mmls for Fell is too small (ef 8 = 0.65 mmls for Fell of Rb). • if the outer doublet corresponds to the Fell-centre, then the peak area ratio of the outer doublet and inner doublet should be 1:2 (ef assuming the reduced form constituted by 1Fell and 2Fe lll centres) but experimentally the observed peak area ratio of the outer doublet and inner doublet is 2:1. It needs an explanation. In fact, the isomer shift 0.45 mmls of the reduced form of the Fe3S4 protein lies between the values of FellI (0.25 mmls) and Fell (0.65 mmls) of rubredoxin (Rb). It indicates that probably, the added electron during the reduction of the oxidised form of the Fe3S4-protein, is shared equally by two ~elll-centres i.e. 3Fe 3+ ~ Fe 3+ , 2Fe 2.S+ (not 2Fe 3+, Fe 2 + ) It leads to the mixed valence state intermediate between ferric (Fe3+) and ferrous (Fe2+). In fact, 0.25 +0.65 the calculated isomer shift ( =- - - - = 0.45 mmls by using the 8-values found for the mononuclear 2 species Rb) value for Fe 2.5+ nicely agrees with the experimental value 0.45 mmls. It supports the fact that in the reduced form of the Fe3S4-protein, the FellI-Fell unit remains coupled to give the average oxidation state +2.5 of iron. (iv) For the Fe4S4-protein (i.e. 4 Fe-4S cubane protein), the MB spectroscopic data are quite interesting. Crystal structure determination cannot give any information regarding the oxidation states

1985

SPECTROSCOPIC METHODS AND OTHER PHYSICAL METHODS IN CHEMISTRY

of iron in the different forms of the Fe4S4 protein. But the MB spectroscopic data can do the task. The isomer shifts (8) for the different forms of Fe4S4 protein are as follows: Fdred (lFelll + 3Fell) Composition: HiPIPox (3Felll + IFell) HiPIP red or Fdox 2 7S III ll (Probably 4 Fe . +) (2Fe + 2Fe ) (Probably 4 Fe2.2S+) (Probably 4 Fe2.S+) 8 (mmls):

0.32

0.42

0.57

(HiPIP stands for high potential iron protein; depending on the Eo values, the Fe4S4 protein can act either as HiPIP or ferredoxin (Fd) i.e.

ra ry

HiPIPox ~HiPIPred or Fdox ~Fdred; £0 (reduction potential at pH ::::: 7.0) = +0.35 V for HiPIP and 0 - 0.6 V for Fd.

These aspects have been discussed in the author's book, Bioinorganic Chemistry.

8 (mnlls): (Calculated value)

3 x 0.25 + 0.65

= 0.35

0.25 + 3 x 0.65 = 0.55 4

0.42

H

0.32

0.57

C

lic

k

(Experimental value)

= 0.45

4

er

8 (mmls):

HiPIPred or Fdox

2 xO.25 + 2 xO.65

e

4

t.m

HiPIPoX< 4Fe2•7s+)

e/

th e

al

ch

em

yl ib

The MB spectra (Fig. 12.4.7.11) of the different forms of Fe4S4 protein show only one signal in each case and it indicates that it does not possess the localised FellI and Fell centres. For example, if Fdox is supposed to bear the 2Fe lll and 2Fe ll centres, then it should show two MB signals with the expected 8-values about 0.25 mmls (for FellI) and about 0.60 mmls (for Fell) in comparison with those of the MB data found for Rb. But it shows one signal with the isomer shift 0.42 moos. The experimentally found isomer shift values can be interpreted by considering the average oxidation state of iron which represents the mixed valence state intermediate between FellI and Fell. In fact, by using the 8-values of Rb, the calculated average 8-values of the different forms of the Fe4S4 protein nicely agree with the experimental values. It strongly indicates that in any form of Fe4S4 protein there is only one kind of Fe-centre.

g en en c

.......................... ~/I "-,,,Cys-8, / ................ 7 I Fe.......... " I "\ ' '"" 8 I \~', ,,/ I II \ \ " X"""

Fe4S4 HiPIPred or Fd ox

..-.., Cys-- 8

Quadrupole split doublet for the

-2

o

+2

+4

I

............ ',

8

lll

delocalised 2Fe -2Fe" coupled centres.

-4

/ '.. . . ', IyS Fe' "

::)

8

.............. ~e/8-·CYS

8

........ ~, I

Fe . "- 8-Cys

Cubane structure (Fe4S4 protein)

Fig. 12.4.7.11 Qualitative representation of the MB spectrum of the oxidised form of Fe4 S4-Fd or the reduced form of HiPIP. constituted By 2Fe III and 2Fe" centres strongly coupled giving rise to the average oxidation state +2.5 per iron.

1986

FUNDAMENTAL CONCEPTS OF INORGANIC CHEMISTRY

It supports the fact that in each fonn of the Fe4S4 protein, all the Fe-centres are equivalent and the equivalent Fe-centres are antiferromagnetically coupled to register an average oxidation state. Conclusions regarding the oxidation state and equivalence of the Fe-centres in Fe4S4 protein:

yl ib

ra ry

The MB-spectroscopic data indicate: • in any fonn of the Fe4S4 protein, only one kind of iron exists and gives one MB signal, • in sharp contrast to the Fe2S2 and Fe3S4 proteins, in Fe4S4, the Fe(ll) and Fe(lll) centres are completely delocalised and they are antiferromagnetically coupled. • average oxidation state of iron is in conformity with the MB data. • magnitude of quadrupole splittings and isomer shifts are the averaged values for a particular combination of the oxidation states of 4 Fe-centres. 24. Alloy and ore analysis: Presence of the MB active elements like Fe, Sn, etc. in any sample can be detected by the MB spectroscopy. Their oxidation states can also be identified from the MB-spectroscopy.

12.5 ELECTRONIC SPECTROSCOPY: UV-VISIBLE SPECTROSCOPY

al

ch

em

Each electronic level is associated with a number of vibrational energy levels and each vibrational level is associated with a set of rotational energy levels. The energy order f