Confacial bioctahedral complexes of molybdenum and tungsten

Abstract

The review deals with the chemistry of confacial bioctahedral or face-sharing bioctahedral (FBO) complexes of molybdenum and tungsten. As shown in the first section on bonding, complexes in which the metals are formally both in a +3 oxidation state are particularly stable owing to the optimal electronic configuration for the formation of metal-metal interactions. Hence the chemistry of such complexes is the most developed and is therefore most emphasized in the review. The fragment approach to bonding, introduced by Summerville and Hoffmann, is used to explain the structural features and reactivity of the FBO complexes. Tabulated data on many of the known complexes are provided, organized by the oxidation states of molybdenum and tungsten respectively. A brief review of the synthetic approaches, especially to M(III)-M(III) systems, is followed by a more lengthy discussion of the reactivity pattern that are emerging. These include interconversion between FBO and other structural types, especially edge-sharing bioctahedral (EBO) complexes, ligand site exchanger reactions and ligand displacement reactions. Some relatively recent chemistry involving reactions of the cooordinated ligands is then discussed, followed by a brief excursion into the known redox chemistry of the M(III)-M(III) systems which can lead to mixed oxidation state complexes. Finally, the possibility of developing catalytic uses for FBO complexes is suggested. The review cites 88 references to original papers and previous review articles. It deliberately avoids extensive discussion of aspects of the field which have been reviewed elsewhere in recent years.