CHM 401

 

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