Dithiadiazolyls as heterocyclic chelators: a radical approach to coordination chemistry?

The ability of heterocyclic compounds to act as chelating ligands via the cleavage of a heterocyclic E-E (or E-E') bond with concomitant formation of M-E (and M-E') bonds is described with particular reference to the 7 pi heterocyclic dithiadiazolyl radicals, RCNSSN. These electron-rich ring systems form a diversity of structural bonding types in which the heterocyclic ring can formally act as a 2e(-), 3e(-),5e(-) or 6e(-) donor ligand. These structural bonding modes are described through examples in which the heterocyclic ring interacts with halide anions, N atoms and particularly metal centres, forming monometallic, dimetallic and trimetallic complexes. The structural features which determine which compounds are likely to act as cyclic chelators are discussed and examples of thiadiazole, diselenadiazolyl and dithiole rings acting as cyclic chelators are given. (C) 1998 Elsevier Science S.A.