Structure—Reactivity Relationships for Ionic Transition Metal Carbonyl Cluster Fragments

Abstract

Many parallels have been made between transition metal clusters and metal surfaces. The geometric features of the metal-cluster-bound ligands are similar to those of adsorbed molecules on a metal surface, and the average metal-ligand and metal-metal binding energies of transition metal clusters are comparable to the binding energies of the metal surfaces and chemisorbed molecules.(1)) The cluster/surface analogy (in terms of chemical properties) begins to break down, however, when the coordination saturation of the transition metal cluster is considered. Typically, the coordination number for metal-ligand interactions of metal clusters is high, and the coordination number for metal-metal interactions is low. Conversely, the high coordination number for metal surfaces is due to the metal-metal interaction. Because most clusters are stable, coordinatively saturated molecules which obey the 18-electron rule,(2) experimental methods for probing coordinatively unsaturated clusters and cluster fragments, e.g., matrix isolation(3) and fast time-resolved infrared spectrscopy,(4) are being developed. The target species for many of these studies are the reactive 17-electron species and higher coordinatively unsaturated species. For the past several years the development of methods for preparing coordinatively unsaturated ionic transition metal cluster fragments has been actively pursued in our laboratory. As a direct result of this work, ionic transition metal cluster fragments have a range of numbers of metal atoms (typicallly 2–8) and ligands (typically CO) can be prepared.