Bonding in cluster compounds

At the moment, all that can be hoped for with such structures is a retrospective rationalization, (b) [ / -C5H5Fe(C0)]4—a tetrahedral cluster with a CO group above each tetrahedron face. The group attached to the closest iron 

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A theory which shows greater applicability to bonding in cluster compounds is the Polyhedral Skeletal Electron Pair Theory (PSEPT) which allows the probable structure to be deduced from the total number of skeletal bond pairs (400). Molecular orbital calculations show that a closed polyhedron with n vertex atoms is held together by a total of (n + 1) skeletal bond pairs. A nido polyhedron, with one vertex vacant, is held together by (n + 2) skeletal bond pairs, and an arachno polyhedron, with two vacant vertices, by (n + 3) skeletal bond pairs. Further, more open structures are obtainable by adding additional pairs of electrons. This discussion of these polyhedral shapes is normally confined to metal atoms, but it is possible to consider an alkyne, RC=CR, either as an external ligand or as a source of two skeletal CR units. So that, for example, the cluster skeleton in the complex Co4(CO)10(RCCR), shown in Fig. 16, may be considered as a nido trigonal bipyramid (a butterfly cluster) with a coordinated alkyne or as a closo octahedron with two carbon atoms in the core. 

Binary carbonyls, containing only metal atoms and CO, are numerous. Some representative binary carbonyl complexes are shown in Figure 13-16. Most of these complexes obey the 18-electron rule. The cluster compounds Co6(CO)j6 and Rh6(CO)x5 do not obey the rule, however. More detailed analysis of the bonding in cluster compounds is necessary to satisfactorily account for the electron counting in these and other cluster compounds. This will be discussed in Chapter 15. 

The theoretical models which have been used to describe the bonding in cluster compounds of the... 

The theoretical models which have been used to describe the bonding in cluster compounds of the main group and transition metal elements are reviewed. The historical development of these models is outlined and special emphasis is placed on those studies which have led to the elucidation of structure-electron count correlations. Theoretical treatments of cluster bonding are based on localised, delocalised (molecular orbital) or free electron methods derived from the solution of the Schrodinger equation for a particle on a sphere. A detailed analysis of the Tensor Surface Harmonic method, as an example of a free electron model, is presented. Group theoretical consequences of the model are also presented. 

The boranes, carboranes, and related compounds are of interest in the field of cluster chemistry. The bonding in cluster compounds is discussed in Chapter 15. 

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