Metallocarboranes vertices

The structure of the bimetallic 10-vertex cluster was shown by X-ray diffraction to be (84). When the icosahedral carborane l,2-C2BioHi2 was used, the reaction led to the first supraicosahedral metallocarboranes with 13- and 14-vertex polyhedral structures (85)-(89). Facile isomerism of the 13-vertex monometallodicarbaboranes was observed as indicated in the scheme above (in which = CH and O = BH). 

To the structural generalizations on carborancs (p. 183) can be added the rule that, in metallocarboranes, the M atom tends to adopt a vertex with high coordination number. M occupancy of a low CN vertex is not precluded, particularly in kineiically controlled syntheses, but isomerization to more stable configurations usually results in the migration of M to high CN vertices. 

The crystal structure of the chromium metallocarborane salt Cs[Cr(BgC2-H9Me2)2] has been determined. The anion consists of two icosahedra sharing a chromium atom as a common vertex. ... 

One particularly interesting category of metallocarborane is that in which a single metal atom is shared between two polyhedra that have a vertex in eommon. In effect, the metal is sandwiched between two nido-carborane residues. Examples are shown in Fig. 17. For such commo compounds, the metal can be assumed to contribute three AO s to the skeletal bonding of each polyhedron, when the (n + 1) rule for closo clusters is found to be obeyed. For example, the isoelectronic... 

Applied to icosahedral carboranes as the starting materials, it leads to metallocarborane based on 13- or 14-vertex polyhedra, e.g. (35, 70),... 

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. 

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. 

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. 

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. 

Fig. 18. Preparative method and proposed structure of one isomer of a fourteen-vertex metallocarborane.

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 original preparation of ten-vertex metallocarboranes involved the deprotonation of the nido-carboranc 6,8-C2B7H13, followed by reaction with a transition metal halide. Additional hydrogen gas was liberated during the complexation, and cZoso-metallocarborane compounds were formed (38) ... 

Ten-vertex metallocarboranes containing both cobalt and nickel in the same polyhedron have been synthesized from the nine-vertex anionic species C5H5C0CB7HS- (Section VIII) by polyhedral expansion (87). The resulting neutral complexes, of formula (C5Hs)2CoNiCB7H8 and con-... 

The chemical reactions of monometallic ten-vertex metallocarboranes have been examined (40). Fricdel-Crafts acylation of C5H6Co(l, 6-C2B7H9) produced a monosubstitution product. Attack occurred on the boron atom farthest removed from the polyhedral carbon vertices. No substitution on the cyclopentadienyl ring was observed. 

The first metallocarborane of this geometry to be synthesized was an unexpected product. In an attempt to prepare a ten-vertex manganese carbonyl complex, the C2B7Hii2 ion, discussed in Section VII, was reacted with BrMn(CO)5. Surprisingly, the only metal-containing compound isolated from the reaction mixture had just 6 boron atoms (36, 50). The course of the reaction may be outlined as follows ... 

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