Eleven-Vertex Metallocarboranes

The first reported preparation of an eleven-vertex closed monometallo-carborane involved the polyhedral expansion of the ten-vertex closo-ear- 

The structure of this compound, inferred from nB NMR spectroscopy (26) is shown in Fig. 19. The metal resides at the high-coordinate vertex, while the 2 polyhedral carbon atoms occupy the two low-coordinate positions in the closed eleven-vertex geometry. The analogous Fe(III) compound was formed using Fe(II) in the polyhedral expansion reaction (24). 

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). 

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. 

The eleven-vertex bimetallocarboranes have primarily been synthesized by application of polyhedral expansion techniques. One obvious synthetic route, polyhedral expansion of a ten-vertex cZoso-monometallocarborane, has been investigated in detail (24). The products arising from this reaction are shown in Fig. 20. Three isomeric eleven-vertex cobaltacarboranes have 

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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. 

The discovery of the thermal metal transfer reaction (27) afforded a third preparative route to bimetallic twelve-vertex metallocarboranes. This method involves the pyrolysis of eleven-vertex cfoso-metallocarboranes or cobalticinium salts of eleven-vertex cowimo-metallocarboranes and results in the production of several isomeric, closed, neutral, twelve-vertex bimetallocarboranes [Eqs. (9) and (10) Fig. 7]. Yields are reasonable in... 

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