Structure and Properties of Metal Clusters and Colloids in Ligand Shells

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In this review article we deal with a particular class of metal clusters, often referred to as the metal cluster compounds or molecular metal clusters. Their main characteristic is to be composed of two parts, an inner core of metal atoms and an external shell of ligand molecules or atoms. In many cases, “interstitial” heteroatoms are also found. The ligand shell is essential for the stabilization of the cluster core and prevents the coalescence of the individual particles to form large metallic aggregates, but in some cases may also contribute to the stability of the cluster. Molecular metal clusters are known in organometallic chemistry since 1930 and earlier [1,2]. It is only in the sixties, however, that molecular clusters of considerable size, containing up to 10-15 metal atoms, have been synthesized and characterized. In the seventies the hope to produce very dispersed catalysts to be used in homogeneous catalytic reactions stimulated intense research activities in this field. In this period clusters containing a few tens of metal atoms were obtained [3]. Soon, it became clear that the chemical stability of the molecular clusters is too high to make them suitable for catalysis. On the other hand, organometallic clusters have become interesting from a different point of view as they are rather well suited for investigating the evolution of various properties from the typically molecular regime of the smaller clusters, to the nearly-metallic regime of large colloidal particles [4].