Non-Bonding Orbitals in Cluster Compounds

Three-dimensional polyhedral molecules such as the borane anions, B Hn , are characterised by a total of n -I- 1 skeletal electron pairs or 4 n -I- 2 valence electrons in total [33]. The isolobal analogy between the BH and ML3 fragments has provided a theoretical relationship between the borane and the transition metal cluster compounds [34]. A detailed analysis of the molecular orbitals of deltahedral transition metal clusters showed that they also have a total of n -h 1 bonding skeletal molecular orbitals, which is the same as that in the borane clusters, and 6 n orbitals that are either non-bonding or involved in the metal-ligand bonding. Therefore, such clusters are characterised by 14 n -I- 2 valence electrons in total. 

It is apparent that deviations from the above electron counts in cluster compounds result from an occurrence of skeletal non-bonding orbitals. The skeletal non-bonding orbitals arise either from topological factors associated with deltahedral geometries or the presence of missing vertices in nido- and arac/ino-structures. 

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