Trigonal prismatic cavities

Fig. 35. Co,3C2(CO)fc, 28, as in its (PhCH2N(CH3)3)+ salt (68). The two encapsulated carbon atoms occupy trigonal prismatic cavities, which share a common vertex [Co(l)]. The two prisms, which are outlined with solid lines for clarity, are rotated with respect to one another (see text), and two further cobalt atoms [Co(4) and Co(5)] assume capping positions. Average Co-Co distance = 2.57 A average Co-C distance = 1.98 A.

As would be expected, there are exceptions to these rules, for example phosphorus has been located in a trigonal prismatic cavity in the cluster [Os6P(CO)ig] f l and arsenic in a square antiprismatic cavity in [RhioAs(CO)22] , even though theoretically it seems unlikely. The aim of this section is to hi ight the different interstitial sites in which main-group atoms have been located, and to illustrate the effects they have on the polyhedral cluster core. 

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. 

On oxidation with FeCla, the trigonal prismatic cluster [Co6C(CO)is] undergoes rearrangement to the distorted octahedral monoanion the carbido-carbon atom is retained within the Cog cavity. Once again, a metal-metal antibonding orbital is implicated in the redox process 191). 

---