Chemistry Explosives Overview Of Metal Chemistry

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Overview of Metal Chemistry Angus Paul Wilkinson School of Chemistry and Biochemistry, Georgia Institute of Technology

Topics to be covered ‹ An

overview of the properties of the metallic elements ‹ s - metals ‹ d - metals ‹ p - block metals ‹ f - block (lanthanides and actinides)

The occurrence of metallic elements

Enthalpies of vaporization

Reactivity ‹ Most

metals are reactive towards reagents such as oxygen ‹ The noble metals, Au, Pt show no tendency to react with oxygen and are generally of low reactivity ‹ Reaction of a metal with a reagent can produce compounds that still display metal-metal bonding

Examples of metal-metal bonding ‹ Rb9O2,

Cs4O

‹ ReCl3,

MoCl2, ZrCl

‹ Fe2(CO)9

Occurrence of cluster formation

S - block metals ‹ Very

reactive

‹ Compounds

tend to be ionic

– much of their chemistry can be explained using the ionic model ‹ Do

not form a wide variety of complexes

‹ Their

chemistry is predominantly that of species in the group oxidation state

Complexes of s-block metals ‹ Complexes

of alkali and alkaline earth cations are restricted to polydentate ligands – crown ethers – cryptands – EDTA and its relatives

Redox properties of s-block ‹

Standard electrode potentials are quite uniform down the groups as the decrease in vaporization and ionization enthalpies is counterbalanced by a decrease in hydration enthalpy

Note that lithium is widely used in the fabrication of batteries. Why?

Crown ethers and cryptands

The macrocylic chelate effect ‹

‹

The efficient binding of chelating ligands is typically argued to be a consequence of entropic effects – Chelating ligand displaces several monodentate ligands

With macrocyclic ligands there is a thermodynamic advantage over more open chelating ligands that is enthalpic in origin

Size selectivity ‹ Crown

ethers and cryptands show quite high selectively for ions that fit the ligand well

Suboxides ‹

A range of alkali metal oxides can be prepared that have the metal in oxidation states of less than one. They are often electrically conducting and can be viewed as materials where O2- is occupying holes in a metallic structure

Rb9O2

Cs11O3

Liquid ammonia solutions ‹ Dilute

solutions of Na in liquid ammonia are blue ‹ Na(s) --(NH3)---> Na+(am) + e-(am) ‹ The color is due to solvated electrons ‹ More concentrated solutions take on a metallic appearance ‹ The solutions are metastable Solvated electrons can also be produced in water based glasses by irradiation, but they are not as stable

Color centers ‹ ‹

Electrons can also be metastably trapped in solid matrices – Called color centers

Irradiation of salts with x-rays or other ionizing radiation produces colored defects. The color of the defect depends on the nature of the host lattice

KCl

KBr

NaCl

In each case the color is from a trapped electron. The color can be rationalized by using electron in a box arguments. As the “box” gets smaller the energy levels get further apart so the absorption moves further towards the blue

Alkalide ions ‹ If

alkali metals are dissolved in alkyamines alkalide ions (M-) are formed – The color of the solution in only dependent upon M-. That cation does not mater.

‹ Alkalides

can be isolated if the counter cation is complexed with a cryptand – Na(2.2.2)+ Na-

Electrides ‹ It

is possible, using macrocyclic, ligands to prepare electrides from solutions of alkali metals – Cs(18-C-6)2+ e-

‹ An

electride is an ionic solid where the anion is an electron

The transition or d - metals ‹ Extraction

of the metals ‹ First row versus heavy transition metals – Stability of high oxidation states – Coordination numbers

‹ Oxocomplexes

‹ Polyoxometallates ‹ First

row versus heavy metals

– Low oxidation state compounds

Extraction of the metals

First row metals versus the heavy transition elements ‹ The

chemistry of the 4 and 5 d metals is similar ‹ The first row TMs differ from the heavy metals – 3 d metals often have oxidizing maximum oxidation states – the heavy metals often display metal-metal bonding when they are in low oxidation states – 3 d metals often show lower coordination numbers than their heavier brethren

Trends in size ‹ In

going across a transition metal row ionic radii tend to decrease ‹ In going down a transition metal group: – ions of the 3 d metal are often much smaller than the 4 and 5 d ions – the 4 and 5 d ions are frequently similar in size

Ionic radii of the 3d metals

Ionic radii down a group ‹ Tetrahedral

Cr(VI) - 40 pm, Mo(VI) - 55 pm, W(VI) - 56 pm ‹ Octahedral Ti(IV) - 74.5pm, Hf(IV) - 85pm, Zr(IV) - 86pm ‹ Square planar Ni(II) - 63pm, Pd(II) - 78pm, Pt(II) - 74pm ‹ Low spin octahedral Co(III) - 68.5pm, Rh(III) 80.5pm, Ir(III) - 82pm

3d metals in high oxidation states

Oxidizing power down a group ‹ Typically,

oxidizing power of highly oxidized species drops on going down a group. High oxidation states for heavy metals are often quite stable.

The maximum possible oxidation states for the halides of TMs ‹ ‹ ‹

Note that the group oxidation state is only achieved in the early part of the transition block Higher oxidation states are often achievable with the heavier metals (4d, 5d) Fluorine often facilitates a higher oxidation state than other halides

Fluorides and oxides ‹ Several

metals only achieve their maximum possible oxidation state in oxides – OsO4 is readily prepared but there is no corresponding fluoride – MnO4- exists but there is no corresponding fluoride – CrO42- is easily prepared but CrF6 is difficult to prepare and is not very stable.

Variation in coordination number on going down a group ‹ The

coordination numbers heavy metal complexes are often greater than those of the 3d metals

Aquo ions ‹ Aquo

ions of 3d metals are common

‹ Aquo

ions of the heavy TMs are rare

– Pd(OH2)4 2+, Ru(H2O)62+

Oxocomplexes ‹

‹

‹

Metals in high oxidation states, particularly at high pH, tend to form oxo or hydroxo complexes rather than aquo species – MnO4-, CrO42-, WO42-, VOL42+

Oxo ligand effectively has a double bond to the metal. There is considerable π interaction between the metal and ligand It is quite common to find the site trans an oxo ligand to be vacant or occupied by a weakly bound ligand due to this π interaction

Oxocomplexes of Ru ‹

Pourbaix diagram shows – high pH favors conversion of an aquo ligand to a hydroxo ligand and conversion of hydroxo to oxo – Increased oxidation state favors conversion of an aquo ligand to a hydroxo ligand and conversion of hydroxo and oxo

Polyoxometallates ‹ Oxo

species of the early transition elements often undergo condensation reactions at low pH to form polyoxometallates ‹ CrO42- + 2H+ -----> Cr2O72- + H2O ‹ 6MoO42- + 10H+ ----> Mo6O192- + 5H2O

Examples

Note: only corner sharing is found for polychromates (built from tetrahedra), but edge sharing of polyhedra is OK for Mo and W (built from octahedra). This is due to distance between metals when tetrahedra and octahedra share edges.

Heteropolyanions ‹ The

polyanions produced on condensation can incorporate non-metal species such as phosphorous and silicon in which case the resulting clusters are referred to a heteropolyanions

Low oxidation state cations of the 3d metals ‹ Low

oxidation state ions of the early 3d metals are either strongly reducing or unknown – Ti2+, Ti3+, V2+, Cr2+

‹ The

(II) oxidation state becomes increasing more stable on going across the row

Low oxidation state ions of the heavy transition metals ‹ The

early heavy transition metals do not form simple compounds with electronegative elements when they are in a low oxidation state – ReCl3 is metal-metal bonded

‹ Low

oxidation states (II/III) become progressively more stable on going across a row

Metal - metal bonding ‹ Many

low oxidation state compound of the heavy transition metals display metal-metal bonds ‹ For the earlier metals these compounds usually involve π donor ligands like halide or alkoxide ‹ For the later metals π acceptor ligands are usually present (CO, PR3 etc.)

Sheet and chain structures ‹ ZrCl

and Sc7Cl10

MoCl2 and it’s relatives ‹

MoCl2 contains octahedral clusters of molybdeum. This motiff is found in variety of compounds

Part of MoCl2 structure

Can be produced from MoCl2 by reaction with HCl. Note faces are capped.

Chevrel phases ‹ Similar

face capped octahedral clusters are found in a family of chalcogenide materials called Chevrel phases – PbMo6S8 etc.

Nb and Ta clusters ‹ Edge

bridged clusters (rather than face capped) with the formula M6X182+ are found for Nb and Ta

ReCl3 ‹ ReCl3

is a metal-metal bonded compound containing Re triangles – Discrete Re3Cl123triangles can be produced by treatment with Cl-

Re2Cl82‹ Classic

example of a species with a quadruple bond

Other metal-metal bonded dimers

Can achieve a wide variety of bond orders!

Nobel metals ‹ Not

readily attacked by aqueous H+ – Difficult to oxidize

‹ However,

there is an extensive chemistry of these elements – Dangerous to assume they are truly inert!

Nobel metal complexes ‹ These

metals occur in a variety of relatively low oxidation states

‹ Many

of their d10 complexes are linear

‹ Many

of their d8 complexes are square

planar

Metal sulfides ‹ The

transition metals become softer on going from left to right across the periodic table ‹ Polysulfide species are quite common ‹ Highly oxidized metal ‘cations’ do not occur as sulfides – FeS2 - is an Fe(II) compound

Monosulfides

Disulfides

The pyrite and CdI2 structures

Applications of layered disulfides ‹ MoS2

– used as a lubricant – used in desulfurization Co/MoS2/Al2O3

‹ TiS2

– battery electrode material – xLi + TiS2 -----> LixTiS2

Group 12 (IIB) ‹ The

elements Zn, Cd and Hg are not classified as transition elements as there is chemistry does not involve valence delectrons

‹ These

elements are not ‘Noble’, like their neighbors – due to low lattice energies

Redox reactions ‹ Zn

and Cd are reactive compared to their neighbors Cu and Ag – Zn2+ (-0.76V), Cu2+ (+0.34V) – Cd2+ (-0.40V), Ag+ (+0.80V)

‹ The

chemistry of Zn and Cs is dominated by divalent cations

‹ Mercury

has a significant chemistry in both (I) and (II) oxidation states

Hg22+ ‹ Hg22+

is a metal-metal bonded species

‹ Mercury

(I) can be easily persuaded to disproportionate

‹ Hg22+(aq)

=== Hg(l) + Hg2+(aq) K = 6 x10-3

‹ Hg22+(aq) +

2OH-(aq) --> Hg(l) + HgO(s) + H2O(l)

The p block metals ‹ Heavier

p-block metals often occur with an oxidation state 2 less then the group value – This is the ‘Inert pair effect’

Group 13 (IIIA) ‹ Aluminum

chemistry is dominated by an oxidation state of (III)

‹ Gallium

and indium form compounds showing both (I) and (III) oxidation states – In(I) and Ga(I) are reducing

‹ Thallium

forms compounds in oxidation states (I) and (III) – Tl(III) is oxidizing

M(III) chemistry ‹ MX3

(M - B, Al, Ga; X = Cl, Br, I) are Lewis acids

‹ For

hard donors the Lewis acid strength is,

– BX3 > AlX3 > GaX3

‹ For

soft donors the Lewis acid strength is,

– GaX3 > AlX3 > BX3

M(I) chemistry ‹ Ga

and In form a number of mixed valent and low oxidation state compounds – GaCl2 is Ga(GaCl4)

‹ However,

some Ga(II) compounds have metal metals bonds – GaS and TMA2(Cl3GaGaCl3)

‹ Tl(I)

has some similarities with both K(I) and with Ag(I) – TlX (X = halides) insoluble – TlOH soluble

Tin and lead chemistry ‹ Both

metals form compounds in oxidation states (II) and (IV)

‹ Sn

(II) is reducing in aqueous solution

‹ Pb(IV) ‹ M(II)

is oxidizing

compounds often have a “stereochemically active lone pair”

Stereochemically active lone pairs

Bismuth ‹ Bismuth ‹ Bi(V)

chemistry is dominated by Bi(III)

is a powerful oxidant

‹ Bi3+(aq) +

3e- ------> Bi(s) E0 = +0.32V

‹ Bi5+(aq) +

2e- ------> Bi3+(aq)

‹ Bi(III)

E0 ~ +2 V

compounds often have stereochemically active lone pairs

Factors favoring distorted geometries ‹ Low

coordination numbers favor a stereochemically active lone pair – BiF3 and SnCl3-

‹ Lighter

elements show a stronger tendency than their heavier relatives to be distorted – Sb > Bi

‹ Small

ligands favor the presence of a distortion

– F and alkoxides rather than I

Lanthanide chemistry ‹ The

lanthanides are technologically important ‹ Their chemistry is simple – dominated by the +3 oxidation state – the M3+ ions are hard

Overview of lanthanide properties

Applications of lanthanides ‹ They

are widely used in ceramics synthesis

– High Tc superconductors – Conducting oxide electrodes

‹ They

are used in phosphors for devices such as TV sets ‹ They are used in lasers – Nd-YAG

Compare oxidation states of the lanthanides and actinides

The lanthanide contraction

Separations ‹

‹ ‹

Lanthanide ions are difficult to fully separate from one another using conventional chemical means as there sizes and charges are so similar – Can separate elements that readily do to +2 or +4

Liquid-liquid extraction used for large scale separation Ion exchange chromatography used when high purity is needed

Complexes ‹ ‹ ‹

‹

The lanthanides cations are all hard and form complexes with hard donor ligands The complexes often have high coordination numbers 6-10 and irregular geometries Ligands interact very weakly with the f-electons – Crystal field splitting is extremely small and often negligible

Complexes have some applications including NMR shift reagents

O

O

(CH3)3C

CF2CF2CF3

fod

Actinide chemistry ‹ Much

more complicated than that of the rare earths ‹ The early actinides can attain a variety of oxidation states

Overview of actinide properties

Applications of actinide chemistry ‹ Nuclear

fuel/ weapons material processing ‹ Nuclear fuel reprocessing ‹ Nuclear waste remediation

Stable oxidation states of actinides

Aqueous solution chemistry of the actinides ‹ The

formation of oxo species is commonplace ‹ UO22+, UO2+ ‹ PuO22+, PuO2+ ‹ The nature of species present in solution is strongly dependent upon both pH and reduction potential

Fission ‹ In

a conventional nuclear reactor 235U nuclei capture neutrons. This leads to the 235U nucleus splitting to give fission products, a lot of energy and additional neutrons which can go on and propagate the nuclear reaction – Fission products span a very wide range of elements

The PUREX process ‹ PUREX

is a widely used chemical process for separating fission products, unused uranium and plutonium from used nuclear fuel