Transition Metal Chemistry Overview

d block chemistry is vast, often thought of in terms of Lewis Acid/Base interactions

Metals act as Lewis acids, which react with Lewis bases

The Lewis acid/base product is generally called a coordination complex and the Lewis bases are called ligands

Wide variety of d-block chemistry to be addressed:

geometries: coordination number (CN = the number of Lewis bases bonded to the metal) can vary from 2 to 8; 4 and 6 are most common

oxidation states: from negative to +7 or +8

bonding: nearly purely ionic to nearly purely covalent; intermediate much more common

reactivity: substitution, addition, elimination, redox - and a number of mechanisms

Structures of Coordination Complexes

CN = 4

Geometries range from tetrahedral (T_d) to square planar (D_4h) with some complexes are known that are between T_d and D_4h (D_2d)

Square planar complexes can exhibit geometric isomerism: ML₂L'₂ cis or trans makes a difference in the chemistry: cis-Pt(NH₃)₂Cl₂ is an anticancer drug, the trans isomer is inert

CN = 5

Two geometries-trigonal bipyramid (D_3h) and square-based pyramid (C_4v); the two geometries are generally close in energy so ligands can scramble.

Berry pseudorotation is seen in mixed ligand complexes, this scrambles ligand positions:

CN = 6

Most common coordination number, octahedral

Often can get distortions from O_h

Extension or compression along one axis : tetragonal : D_4h

Change along two axes : rhombic : D_2h

Change along the diagonal (the C₃ axis) : D_3d

Substitutions:

disubstituted ML₄L'₂ : cis (C_2v) or trans (D_4h)

trisubstituted ML₃L'₃ : fac (C_3v) or mer (C_2v)

CN = 7, 8

Very rare for first row (steric), found occasionally in 2nd or 3rd row complexes

Ligands

Usually ligands are Lewis bases (nonmetals, either anions or neutrals with lone pairs) but this is not required.

The ligand could be another metal complex to give a multimetal compound

Re₂Cl₈^2– (D_4h)

(quadruple metal-metal bond)

Traditional ligands have N, O, halide, S, P as the binding site (all have lone pairs)

Binding of C to metals is also well known - this is organometallic chemistry

Ligands may have multiple binding sites:

monodentate : one binding site NH₃, Cl^–, H₂O, CN^–

ambidentate : two available binding sites but only one can be used CN^–, SCN^–.

This gives the possibility of linkage isomerization : M-SCN differs from M-NCS. To indicate which atom in an ambidentate ligand binds to the metal the notation used is ligand name-κbinding atom (κ = Greek letter kappa); for example for SCN^– bound through the S atom the naming convention is thiocyanato-κS ...

bidentate : two binding sites and both used

ethylenediamine (en), oxalate (ox), 2,2'-bipyridine (bipy), acetylacetonate (acac), phenanthrene (phen)

chelation : ligand plus metal form a ring system; requires a multidentate ligand

Chelation leads to the possibility of chirality in a coordination complex:

For example: [Ni(en)₃]²⁺ has two possible enantiomers

The easiest way to determine if a molecule or ion is chiral or not is to identify the point group: if the point group does not contain a mirror nor an inversion, then the molecule or ion is chiral. If σ or i is present, then there is no chirality.

tridentate : three binding sites used

H₂NCH₂CH₂NHCH₂CH₂NH₂ (dien)

quadridentate : four binding sites

porphyrin

polydentate : many binding sites

EDTA^4– (^–O₂CCH₂)₂–NCH₂CH₂N–(CH₂CO₂^–)₂

binds strongly to nearly all metals; used to treat heavy metal poisoning

Thermodynamics

Simple coordination complexes are generally synthesized by adding ligands to a solvated metal ion, typically using water as a solvent:

[M(OH₂)₆]^m+ + nL ⇄ [M(OH₂)_6–nL_n]^m+ + n H₂O

The equilibrium constant for the reaction is K_eq = β_n = [[M(OH₂)_6–nL_n]^m+]/[[M(OH₂)₆]^m+][L]ⁿ

The extent of reaction, i.e. the maximum value for m, depends on the ligand, solvent, and metal ion.

By using titration techniques, the stepwise equilibrium constants, called formation constants, can be evaluated:

[M(OH₂)₆]^m+ + L ⇄ [M(OH₂)₅L]^m+ + H₂O     K_f1 = [[M(OH₂)₅L]^m+]/[[M(OH₂)₆]^m+][L]

[M(OH₂)₅L]^m+ + L ⇄ [M(OH₂)₄L₂]^m+ + H₂O     K_f2 = [[M(OH₂)₄L₂]^m+]/[[M(OH₂)₅L]^m+][L]

[M(OH₂)₄L₂]^m+ + L ⇄ [M(OH₂)₃L₃]^m+ + H₂O     K_f3 = [[M(OH₂)₃L₃]^m+]/[[M(OH₂)₄L₂]^m+][L]

etc.

Then β_n = K_f1K_f2...K_fn

Dissociation constants are the inverse of formation constants, i.e. K_d1 = 1/K_f1, etc.

Based on statistical considerations K_fn > K_fn+1

Deviations from this prediction indicates structural or electronic effects need to be considered.

Possible reasons:

Chelation

Macrocycles

Steric Repulsion

Change in Coordination Number

Electronic Delocalization (comparable to conjugation in organic chemistry)