Crystallographic studies, transition metal clusters

There has been considerable work in the literature on the structure of very small particles and clusters. Interest in this field has been primarily due to Ino s (1966) early experimental studies of normally fee metals prepared by vapour condensation which showed that a sizable portion of the particles exhibited non-crystallographic structures. These non-crystallographic atomic clusters or polycrystalline nuclei have been observed to consist of pentagonal bi-pyramid or icosahedra form of twinned structures and are known as multiply twinned particles (MTPs). EM studies of supported transition metal catalyst systems have indicated that MTPs sinter faster in catalytic reactions leading to the loss of surface area and are not beneficial to catalysis (Gai 1992). We describe the structure and the role of MTPs in catalysis in the following sections. 

The active site of methionine aminopeptidase contains a binuclear cobalt complex that is required for activity, although a number of divalent metal ions support turnover to varying degrees. X-ray crystallographic studies on the enzyme in complexes with transition state analogs suggests that the binuclear metal cluster serves to stabilize the tetrahedral intermediate in peptide hydrolysis. ... 

Predicted structures of transition metal-carbonyl complexes using Wade s rules are often, but not always, accurate.33 For example, the clusters M4(CO)12 (M = Co, Rh, Ir) have 60 valence electrons and are predicted to be nido complexes (14 + 4 valence electrons). A nido structure would be a trigonal bipyramid (the parent structure) with one position vacant. X-ray crystallographic studies, however, have shown these complexes to have tetrahedral metal cores.34... 

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