Didentate ligands bridging

Freedom to rotate about the C—C bond in diaminoethane permits cis or trans isomers, capable of chelation and bridging respectively (top). For rigid diaminobenzene (bottom), rearrangement is not possible, and the two isomers shown have exclusive, different coordinating functions as didentate ligands. 

Pyrazine (L39) and substituted pyrazines have long been known to act as exo-didentate ligands to linearly bridge metal ions, generating oligomeric and polymeric metal complexes with infinite chain and... 

Condensation of ketones or aldehydes with the methyl ester of dithiocarbazic acid produces hgands with S,N,0 or S,N donor sets. An example is (142), which acts as both a didentate and a tridentate to ReO. Nickel(II)-templated reactions of pentanedione or salicylaldehyde with aminoethanethiol produce N,0,S ligands such as (143) and (144) these form simple mononuclear complexes or else dinuclear species with thiolate bridging two Ni centers. Mixed donor Schiff-base hgands including other heteroatoms such as P (145) and Te (146) have appeared. 

The solid state structure of H3Zr2(PMe3)2(BH4)5 (compound A) is shown schematically below. There are four tridentate and one didentate BH4 and three bridging hydride ligands. 

---