Cluster Solid State Chemistry: A Frontier Discipline Between Metallurgy and Molecular Chemistry

Abstract

Many compounds with early transition metals (Ti, Zr, V, Nb, Ta, Mo, W, Re...) in low oxidation states are known which are characterized by metal - metal bonds. These metals form aggregates, called clusters, such as pairs, triangles or regular shaped polyhedra such as tetrahedra, octahedra or polyoctahedra. These clusters are surrounded by ligands such as oxygen, halogen or chalogen atoms. In the case of suboxides, subhalides or chalcogenides where the formal oxidation state of the metal atoms is between 3 and 4, these clusters consist of pairs, triangles, rhomboids, zigzag chains or tetrahedra and are surrounded by ligands stacked in a close packed manner. On the other hand, the cluster compounds where the metal atoms have a formal oxidation state between 1 and 3 are built up of octahedral or polyoctahedral metal clusters which, surrounded by ligands, form “cluster units”. The structures containing these cluster units are often arranged as the atoms in NaCl, CsCl or ReO3 structure types. The number of valence electrons available tor the metal-metal bonds is related to the transition metal/non-metal ratio and increases with the degree of condensation of the cluster (or nuclearity = number of metal atoms in the cluster).