Twelve-Vertex Metallocarboranes

Monometallic Complexes with Identical Carborane Ligands 

The d3 chromium (III) complex (1,2-C2B9Hu)2Cr- was prepared (85) from 7,8-C2BgHn2- and CrCl in THF solution. An X-ray crystal structure determination on the Cs+ salt, performed by St. Clair, Zalkin, and Templeton (90) showed it to have a symmetrical sandwich structure in which the 

The most intriguing property of these tetravalent metal complexes wras 

2 Molecular orbital theory has recently been employed by Wegner to account for these structural changes [P. A. Wegner, Inorg. Chem.., 14, 212 (1975)]. 

Limited investigations of the chemical reactions of metallocarboranes have been reported. In general, these complexes are much less reactive than the analogous metallocenes, yet several unique new species have been prepared from the unsubstituted metallocarboranes. 

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It should be noted that in the polyhedral expansion process, as idealized in Eqs. (5) and (6), the product mctallocarborane has one more vertex than was present in the carborane starting material—hence the origin of the descriptive phrase polyhedral expansion. By contrast, wThen metallocarboranes are prepared by reaction with O2B9H112- ions, which are prepared from the icosahedral C2B10H12 carboranes, twelve-vertex metallocarboranes result. 

Several other twelve-vertex metallocarborane carbonyl complexes have been prepared (51). In general, the chemistry and structures of these species, when investigated, have been found to parallel the analogous cyclopentadienyl metal carbonyls. 

Examples of bimetallic twelve-vertex metallocarboranes have been provided by various synthetic efforts. The preparation of bi- and trimetallic chain complexes by polyhedral subrogation of (1,2-C2B9Hn)2Co has been mentioned earlier (38, 34), and the Co(C2B8Hi0)Co fragment present therein may be considered as an example of this type of bimetallic complex. 

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... 

Only one trimetallic twelve-vertex metallocarborane has been reported. This species, (C5H5)3Co3C2B7H9, arose as a side product during the polyhedral expansion of 2-C5H6-2-Co-l,6-C2B7H9 with Co(II) and C6H5 (25, 28). The isolation of this trimetallic complex suggests that the polyhedral expansion reaction may be extended to bimetallic substrates to produce novel metal-rich polyhedra. 

Our approach to the subject has been to divide the metallocarboranes according to the size of the polyhedron. Starting with twelve-vertex compounds, which constitute the majority of the effort, we proceed to the larger polyhedra, so far unknown in the B H 2 and C2B 2H series, and then to the lower polyhedra. Further subdivisions within each polyhedral size include synthesis, structures, and properties of monometallic complexes, reactions of monometal lies, bimetallic preparations and reactions, and, in two instances, trimetallic compounds. 

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. 

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