Metal atom carboranes

To conclude, we shall mention some metal-atom reactions with boranes (172) and carboranes (173). When cobalt atoms reacted with pentaboraneO) and cyclopentadiene, a number of new metalloborane clusters were formed (172), two of which were 65115003(17-05115)3 and cyclopentyl-B5H40o2(i7-05H5)3. Possible structures for the former are shown in Fig. 42. The reaction of cyclopentadiene, pentaboraneO), and 2-butyne with cobalt atoms yielded the metallocarborane species illustrated in Fig. 43 (173). 

Neutral carboranes and boranes react with transition-metal complexes forming metallocarboranes or metalloboranes, respectively. However, most metallocarboranes and metalloboranes are prepared from transition-metal halides and anionic carborane and borane species ( 6.5.3.4) or by reacting metal atoms and neutral boranes and carboranes. These reactions are oxidative addition reactions ( 6.5.3.3). 

Our discussion will be restricted to metallacarboranes in which the r/-block metal is incorporated into the polyhedral framework of the carborane. No attempt will be made to cover those compounds where the transition metal atom is in a bridging group linking several carborane polyhedra together or is involved solely as a member of a substituent group. Since much of the insight into the chemistry of the metallacarboranes is obtained from structural and bonding... 

Ru3(CO)i2] and 7,8-Me2-Kfframework bridges a triangular arrangement of ruthenium or osmium atoms with the open CBBBlf or CCBBB faces, respectively, coordinated in a pentahapto fashion to one metal atom while the carborane cage forms two exo-polyhedral B-H M bonds with the other two metal atoms. 

The material reviewed in this Chapter hitherto has focused on metallacarboranes in which the metal atom is a vertex in an icosahedral cage framework. Until recently, monocarbollide metal compounds with core structures other than 12 vertexes were very rare since suitable carborane precursors were not readily available." However, Brellochs recent development of the reaction of decaborane with aldehydes to give 10-vertex monocarboranes permits a considerable expansion in this area of boron cluster chemistry. As a consequence, several intermediate-sized monocarboranes are now easily accessible and we have recently begun to exploit the opportunities that these present. In particular, we have focused thus far on complexes derived from the C-phenyl-substituted species [6-Ph- zJo-6-CBgHii] It is clear from these initial studies that a wealth of new chemistry remains to be discovered in this area, not only from among the metal derivatives of PhCBg car-boranes such as those discussed in this section, but also in the metal complexes of other newly available carboranes. 

Some c/o o-Mbtallo-boranbs and -carboranes Containing at Least 1 Metal Atom in the n-VBRTEX Polyhedral Cluster... 

Although most known metallocarboranes have only 1 metal atom per polyhedron, the existence of an increasing number of metallocarboranes with 2 or more metal atoms per cluster emphasizes the close relationship between metal clusters and borane clusters. Since they can be synthesized from cZoao-carboranes by replacing BH units by metal-carbonyl or metal-cyclopentadienyl residues, carborane clusters can effectively be used as disposable templates on which to fabricate... 

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... 

By no means do all metallocarboranes have the metal atoms occupying vertices of the basic polyhedra. Apart from many derivatives in which o-bonded metal residues occupy exo sites attached to particular skeletal atoms, several metalloboranes and -carboranes are known in which the metal occupies an edge-bridging site, effectively replacing a bridging hydrogen atom of the parent borane. Many are nido species related to BeHio, for example, the /x-silyl and /i-germyl carboranes. 

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. 

This situation has forced us to compare the experimental data with more-or-less empirical hypotheses, each one of which has its advantages and has enjoyed some popularity. So far the following four different approaches have been proposed (a) noble gas rule 112) (b) topological hypothesis 88) (c) relationship between metal clusters and polyboranes and carboranes 120, 121) and (d) LCAO-MO treatments limited to the metal atoms (24, 26, 34,85,105, 111). 

Recently, Wade has pointed out a formal analogy between the electronic structures of carboranes and polyboranes and those of metal carbonyl clusters based on the assumption that certain triangulated polyhedra require the same number of skeletal orbitals whether there are BH or CH units as well as transition metal atoms at their apices (120). This assumption is quite reasonable as the synthesis of a large number of polyboranes and carboranes in which a transition metal atom takes the place of a BH skeletal unit may be carried out (70). 

For a molecular skeleton consists of m transition-metal atoms and 2 main-group atoms, such as transition-metal carboranes, its bond valence is... 

Many boranes and carboranes can act as very effective polyhapto ligands to form metallaboranes and metallacarboranes. Metallaboranes are borane cages containing one or more metal atoms in the skeletal framework. Metallacarboranes have both metal and carbon atoms in the cage skeleton. In contrast to the metallaboranes, syntheses of metallacarboranes via low- or room-temperature metal insertion into carborane anions in solution are more controllable, usually occurring at a well-defined C2BK open face to yield a single isomer. 

Grimes153 have prepared 11-13 atom nido-cages by the insertion of transition metal atom moieties into carboranes. Recently, 13 and 15-vertex m rfo-carboranes have been prepared by Hosmane and Grimes1S4) by the oxidative fusion of carboranes and borane anions. 

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