Metal Cluster Molecules: From Molecule to Metal

Abstract

Metal cluster compounds (MCC-’s) present an interesting class of cluster solids which is very suitable to study the transition to “metallic behavior” of a metal cluster as its size increases [1]. An MCC consists of identical macromolecules (the metal cluster molecules), which can be ionic or neutral and which are composed of a metal core (cluster) containing a given number (n) of metal atoms. The core is surrounded by a ligand “shell” formed by ligand atoms (Cl, I, 0,… or ligand molecules (CO, PPh3,… which are chemically bonded to metal atoms at the surface of the metal cores. Since chemical compounds are involved, a sample of a given MCC contains only one particular type of macromolecule (provided that it is pure, of course), and thus presents a macroscopically large collection of identical metal clusters, mutually separated by the ligand shells (plus the counterions in case of the ionic forms). Thus the ligand shells provide a highly effective means of “chemical stabilization” of the small metal particles (analogous to metal colloids). In going from one compound to the other, the type of metal atom in the clusters or the size n of the clusters can be varied. At present a few hundreds of MCC-’s are known already, most of them with cluster sizes ranging up to n = 20 − 30, and with many of the transition metal elements (Fe, Co, Ni, Mo, Ru, Rh, Pd, Ag, Os, Ir, Pt, Au,… For some elements even giant MCC-’s are known, the metal core size becoming as large as n = 309 for Pt and n = 561 for Pd. Clearly, such sizes are already closely approaching those of the smallest metal particles in metal colloids (n ≃ 103 − 104 atoms). Not surprisingly therefore, these giant MCC-’s offer very promising possibilities for the study of the physical and chemical properties of catalysts. In particular the comparison of the properties of the ligated and the bare metal clusters (obtained by removing the ligand shells) are of considerable importance in this regard.