Clusters in ligand shells

Before discussing various high-nuclearity clusters and cluster compounds we want to address the general question of what can be defined as a metal cluster. To this end we consider two examples of low-nuclearity Au compounds. According to a widely used definition, a metal cluster is characterized by direct chemical bonds between metal atoms.Here, we want to correlate the metal-metal distance in the cluster with the strength of the metal-metal bond. 

Trigold oxonium cations consist of three Au-phosphine units boimd to an oxygen center in a pyramidal geometry (Fig. 6). This compound had been synthesized from dilferent phosphine ligands of varying steric bulk. For large phosphine ligands, 

In conclusion, analysis of bond distances is a useful tool for describing the bond character in a polynuclear complex. Other examples of this approach are given below. 

Gunter Schmid, Giuliano Longoni and Dieter Fenske 

In the sixties and the seventies. X-ray crystallography, computer science and NMR spectroscopy had advances in a major impact on cluster chemistry. Discoveries of unprecedented structural and spectroscopic features became an almost routine occurence, and cluster chemistry started to blossom, while the frontiers of the area were continously moved ahead. 

It is not the aim here to completely review all the past findings, but rather to concentrate on a few leading examples and to restate the present state-of-the-art through a concise, if often rough, summary encompassing the miscellaneous facets of the chemistry of low-valent organometallic clusters. 

1 Interplay Between Electronic and Steric Factors in the Growth of B ansition Metal Molecular Clusters in Ligand Shells 

Tfable 3-1. Comparison between the total number of required and allowed carbonyl groups around a metal octahedron of increasing frequency/  

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Longoni, G. Iapalucci, M. C. Clusters in ligand shells. Low-valent organometallic clusters. In Clusters and Colloids From Theory to Applications Schmid, G., Ed., VCH New York, 1994 p 91. 

General introduction/Electronic structure of metal clusters and cluster compounds/ Clusters in ligand shells/Clusters in cages/Discrete and condensed transition metal clusters in solids/Chemistry of transition metal colloids/Perspectives G.A. Somoijai, Introduction to Surface Chemistry and Catalysis, Wiley New York, 1994... 

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