TRANSITION-METAL CLUSTERS IN HOMOGENEOUS CATALYSIS

This chapter discusses catalytic applications of transition metal clusters in a homogeneous phase. Transition metal clusters may simply be defined as discrete molecular units containing at least three metal atoms, in which metal–metal bonding is present. Transition metal clusters can be designed in various geometries and different metals can be joined together to accommodate specific reactivity patterns. Most transition metal clusters are soluble molecular compounds that react in a homogeneous phase with substrates and can be isolated and characterized like mononuclear metal complexes. The catalytic applications quoted in this chapter are regarded as cluster based reactions as far as these reactions are catalyzed on addition of a transition metal cluster. Whether the cluster is a catalyst precursor or a catalytically active species is not a crucial point. No distinction is made between homogeneous catalysis by mixed-metal clusters and homogeneous reactions catalyzed by mixtures of transition metal clusters because the nature of the synergistic effects involved has never been elucidated in a convincing way. The challenge in cluster catalysis appears to reside in the search for novel catalytic reactions and for highly selective catalytic processes, making use of the unique polymetallic coordination sites in soluble organometallic molecules. © 1993, Academic Press Inc.