Metal polyhedron cavity

The structures of 204-207 have been established in the solid state by single crystal X-ray diffraction. Clusters 205 and 206 are based on the Ru6C octahedral skeleton. The [2.2]paracyclophane ligand in 205 coordinates over a triangular metal face, and in 206 one [2.2]paracyclophane adopts a similar face-capping bonding mode and the other coordinates in a terminal mode to an apical Ru atom. The metal polyhedron in 204 is relatively open in comparison to the octahedron having only nine Ru-Ru bonds. A carbide atom occupies the central cavity and interacts with five of the six Ru atoms. 

Fig. 43. [Re,C(CO)24p, 35, as in its Et4N+ salt (77). The Re, polyhedron comprises a trans-bicapped octahedron, with the carbide carbon at the center of the octahedral cavity (mean Re-C = 2.12 A). Re- Re bond lengths average 2.993 A within the octahedron and 2.970 A for bonds to the capping atoms. There are three terminal carbonyls per metal atom, and the anion has overall D d symmetry.

A peculiarity of the three-dimensional clusters, containing more-or-less regular polyhedra of metal atoms, is the existence of a central cavity whose dimensions, as shown in Table III, are a function of the particular geometry of the polyhedron. The existence of such a hole is confirmed by the formation of a large number of carbide derivatives. 

Finally, if even larger main-group elements, e.g. Sn and Sb, are to be fully encapsulated within a cluster polyhedron, it follows that even larger cavities are required. Steric effects suggest that because the fourth-row main-group elements have covalent radii of a similar size to those of most platinum-group transition metals. 

It is useful to start the description of the electronic structure of a ligated metal cluster by separately considering the bare metal atom duster, with the exact same geometry as the metal core in the ligated duster, and the ligand polyhedron, as an empty spherical cavity, and then finally allowing the two parts to interact (Fig. M6). 

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