Polyhedral vertex sites

If the square pyramidal metal carbonyl carbides Fe5(CO)i5C ° and Os5(CO)i5C are treated in a similar manner to I xyi ( ()) i T that is, as clusters in which all four of the core carbon atom s valence shell electrons are used for skeletal bonding, then they are seen to have the expected nido shapes of systems with five skeletal atoms (the metal atoms) held together by seven skeletal bond pairs. By contrast, if these carbide carbon atoms had occupied polyhedral vertex sites, with a lone pair of electrons in an exo-oriented sp hybrid orbital, then the number of skeletal bond pairs would have been reduced by one and the number of skeletal atoms would have increased by one. The five metal atoms and the carbide carbon atom would have had to be accommodated in some way on a trigonal bipyramidal skeleton. Clearly, the assumption that all four valence shell electrons from the carbide carbon atom are involved in the skeletal bonding is vindicated. 

The nido eleven-vertex complex, C5H5C0C2B8H12, has been prepared, and spectral data indicate the presence of B—H—B and B—H—Co bridges (67). The commo eleven-vertex metallocarborane, (C2B8Hio)2Co-, prepared by polyhedral expansion of 1,6-C2B8Hio with Co(II) in the absence of cyclopentadienide ion, has carbon atoms in positions identical to the sites in the C5H5C0C2B8H10 isomer prepared by polyhedral expansion in the presence of C5H5-, i.e., at low-coordinate vertices (26). 

A novel ten-vertex bimetallocarborane, (CsHo C C BeHs, is produced upon the polyhedral expansion of 1,7-C2B6H8 (16). The expected monometallic nine-vertex species, C5H5CoC2B6H8, is also produced in this reaction (see Fig. 4). An X-ray crystal structure of the bimetallic product (61) showed the 2 cobalt atoms to occupy adjacent positions on the two equatorial belts of the bicapped square antiprism. The carbon atoms occupy the low-coordinate caps. Thermal rearrangement (30) produced an isomeric complex in which the metals are separated from each other, but the carbon atoms remain in the low-coordinate sites. 

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