Variations characteristic of metal clusters

Although the principles of cluster bonding developed for main-group clusters carry over to transition-metal clusters of the group 8/9 metals with carbonyl ligands, we fully expect transition metals to exhibit variations on this cluster bonding theme as well as novel behavior not seen in main-group systems. In this section we introduce those aspects of cluster chemistry characteristic of transition-metal clusters. 

A cluster with an interstitial atom requires fewer external ligands. The total number of external ligands are limited by the space available on a cluster surface. Hence, although [Fe6C(CO)i6]2 is known, 11 CelCO) 1 hI2 is not. On the other hand, the larger surface of the Ru analog permits both [Ru6C(CO)i7] and [Ru6(CO)i8]2- to be 

The eve count, which includes the external ligands, hides the similarity in cluster bonding counts that exists for capped and uncapped clusters. The simple octahedral cluster has 14n + 2 = 86 eve whereas the capped octahedral cluster possesses 98 eve, i.e., 14 x 6 + 2 + 12. The correction is numerically the same but in the opposite sense to that necessary when a single vertex of a closed cluster is left vacant, i.e., in the case of capping one adds the non-bonding and ligand electrons associated with the added vertex. It follows that one adds 12 electrons to the eve count of the primary cluster for each cap present. 

In8R6 which consists of a In6R.4 octahedron (bare In atoms trans) capped by two RIn fragments in a transoid geometry. Formally the RIn fragments contribute two electrons each and the In contribute one if a lone pair is assumed, yielding sep = 7. 

Exercise 3.6. A cluster is found to have the molecular formula [Os+CO ]2-. Generate a set of likely cluster geometries. 

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