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

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

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

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

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