CHM 501 Lecture


Transition Metal Chemistry

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:

Structures of Coordination Complexes

CN = 2, 3

Rare, follow VSEPR rules CN =2, linear [Ag(NH3)2]+; CN = 3, trigonal planar

CN = 4

Quite common; found in two geometries-tetrahedral (Td) and square planar (D4h) some complexes are known that are between Td and D4h (D2d)

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

Not rare, not common; two geometries-trigonal bipyramid (D3h) and square-based pyramid (C4v); 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 Oh

Extension or compression along one axis : tetragonal : D4h

Change along two axes : rhombic : D2h

Change along the diagonal (the C3 axis) : D3d

Substitutions:

disubstituted ML4L'2 : cis (C2v) or trans (D4h)

trisubstituted ML3L'3 : fac (C3v) or mer (C2v)

CN = 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

Re2Cl82– (D4h)

(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

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

tridentate : three binding sites used

H2NCH2CH2NHCH2CH2NH2 (dien)

quadridentate : four binding sites

porphyrin

polydentate : many binding sites

EDTA4– (O2CCH2)2–NCH2CH2N–(CH2CO2)2

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