Trigonal prismatic/octahedral interconversions

Related work proved the presence and determined the source of the interstitial nitride in the isoelectronic cluster [Rh6N(CO)i5] , and the facile interconversion of the trigonal prismatic and octahedral clusters (Equation 3). ... 

Analogous considerations apply to interconversions of MX5 molecules between trigonal bipyramidal structures via, for example, square pyramidal ones, or to interconversions of MXg molecules between octahedral structures via trigonal prismatic ones (see Chapter 8), and, quite generally, to all interconversion processes between isometric structures. A more rigorous treatment of these problems would have to take into account the concept of the permutation-inversion group [23]. 

The interconversion between octahedral and trigonal prismatic is of relatively high energy because an octahedron has 12 metal-metal bonds and a trigonal prism has only nine metal-metal bonds and, therefore, three bonds must be broken or formed. Such processes are quite rare and examples of reversible processes are limited to a few carbide clusters of the Group 9 metals. 

Cobalt and rhodium also form a series of high nuclearity carbido-cluster compounds. Contrasting with carbido compounds of the iron group which have a clear tendency to put the carbide atom in octahedral cavities, carbides of the group 9 often place it in trigonal prismatic cavities. As shown in scheme in Fig. 3.12, the parent compound [Co6(CO)i5C] of a series of encapsulated car-bido-cobalt species may be prepared by the reaction of Co3(CO)9CCl with [Co(CO)4]. The same scheme also describes some carbido-cobalt cluster interconversions. Rhodium carbide clusters are in general similar to the cobalt ones. 

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