Structure of carbonyl clusters

An obvious problem that was of interest to workers in the field of metal carbonyl clusters was the prospect of predicting the structures of the compounds obtained. The principle of the attainment of the inert gas configuration by the metal atoms was one of the guiding rules applied in all the early considerations of the structure of these compounds. A major development occurred when Wade drew an analogy between the structures of the metal carbonyls and boron hydride compounds whose structure were well established. Using this approach Wade was able to predict the correct structure for a number of polynuclear metal carbonyls many of which had been rationalised in terms of the effective atomic number rule. One of the early successes of the Wade approach was the structure of [Os6(CO)ig], Fig. 2, which was 

These and related methods of structure prediction depend upon the allocation of specific electron counts to different framework geometries and these applications have been very successful in assigning structures to cluster carbonyls and their derivatives. However, for the higher polynuclear carbonyls as the molecularity of the compounds increases the predictive power of the theories becomes less decisive in differentiating between alternative structures. In essence the Wade-Mingos approach assumes that the frontier orbitals of the complex primarily involve metal orbitals so that any variation in the electron occupation will be reffected in a structural change in the metal framework. The Wade theory also requires that the structure of the complexes are based on triangulated polyhedra as found for the boron hydrides. 

For structures of higher nuclearity with a number of different triangulated metal faces the capping of the metal polyhedron can clearly occur in more than one way. 

Thus for the ions [H40sio(CO)24] and [Osio(CO)26] , which on the basis of electron counting should have the same structure, and from Wade s theory would be based on a central octahedral array of osmium atoms, the capping of the trigonal faces of the basic octahedron occurs in two different way leading to the structures shown in Fig. These polyhedra may be derived from the structure of the [HOs9(CO)24], see Fig. by capping the faces Os(2), Os(5), Os(6) or the Os(4), Os(6), Os(l), respectively. 

Clearly there is a necessity to consider the basis for the eleetron eounting methods that have been applied to these type of compounds and in particular the approach to higher nuclearity clusters. There is little doubt that the initial theories that were successful with compounds of low metal nuclearity were an important stimulant in the development of the subject, however some of the more recent developments in this area of chemistry involve the preparation of clusters of higher metal nuclearity and new theoretical approaches to their structure and properties will be necessary. 

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