Metallocarboranes polyhedral contraction

Polyhedral contraction is not so general a method of preparing metallocarboranes as is polyhedral expansion since some metallocarboranes degrade completely under these conditions. 

The polyhedral contraction route to metallocarboranes consists of the degradative removal of a polyhedral boron atom of a metallocarborane followed by oxidative closure of the resulting m do-metallocarborane complex to a closo species having one fewer vertex than present in the starting material (68) ... 

Synthetic polyhedral subrogation for the preparation of polymetallo-carboranes from monometallocarboranes is an offshoot of polyhedral contraction in that, after degradative removal of a BH vertex, a transition metal ion is reacted with the m do-metallocarborane produced rather than with an oxidizing agent. In this way, a new transition metal vertex is incorporated into the polyhedral framework without a change in the number of vertices between reactant and product (Fig. 6) ... 

Degradation of the icosahedral 1,2- and 1,7-C2Bi0Hi2 isomers with strong base to produce the nido eleven-vertex anions 7,8- and 7,9-CoB9Hi2-has been previously discussed and is an important route to the preparation of twelve-vertex monometallocarboranes. The discovery that similar reactions could be performed on metallocarboranes led to the isolation of novel chains of metal atoms bridged by carborane groups and to the development of the polyhedral contraction and polyhedral subrogation reactions. 

Reactions of monometallic eleven-vertex metallocarboranes have been discussed in previous sections and may be summarized briefly as (a) polyhedral expansion to bimetallic twelve-vertex complexes and (6) thermal metal transfer to bimetallic twelve-vertex compounds. Polyhedral contraction to ten-vertex monometallocarboranes is discussed in Section VII. 

There are now at least a dozen synthetic routes to metallocarboranes including (i) coordination using nido-carborane anions as ligands, (ii) polyhedral expansion reactions, (iii) polyhedral contraction reactions, (iv) polyhedral subrogation and (v) thermal metal transfer reactions. These first five routes were all devised by... 

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