Mixed valence complexes involving MM quadruple bonds (M=Mo or W).

The MM quadruple bond of configuration MM sigma2pi4delta2 is redox active and in many ways ideally suited for studies of mixed valency when two or more such centres are linked by a bridging ligand. In this account, the mechanism of electronic coupling is examined for complexes of the type [L3M2bridgeM2L3]0/+ where L, a pivalate; bridge, a dicarboxylate or related ligand and M, Mo or W. The represented examples allow us to probe electronic factors close to the class II/III border and readily distinguish between electron and hole transfer in the superexchange mechanism. The potential for mixed valence organic radical anions mediated by the M2 centre is also raised and one specific example of class III behaviour is described.