Polyhedral metallaboranes

Main Group Element Metallaboranes. A variety of metallaborane clusters, which incorporate main group metals in vertex positions of polyhedral metallaborane clusters, have been reported. Examples are (BH4)BeB5HlfJ (165), MgB1QH12 (166), [(CH3)HgB10H12] (167),... 

Kennedy, John D., The Polyhedral Metallaboranes, Part I Metallaborane... 

A large number of metallacarboranes and polyhedral metallaboranes of s-, p-, d-, and f-block elements are known. In these compounds, the carboranes and boranes act as polyhapto ligands. The domain of metallaboranes and metallacarboranes has grown enormously and engenders a rich structural chemistry. Some new advances in the chemistry of metallacarboranes of f-block elements are described below. 

In the area of synthesis, the structural and theoretical framework contributed by Lipscomb provided a foundation for the monumental synthetic work of M. F. Hawthorne and his students, who transformed boron cluster chemistry first by their synthesis of the polyhedral borane anions BioHio and Bi2Hi2 (30) and later by the synthesis of metallacarboranes (31). In our own laboratory, considerations of charge distribution in borane anions (14) prompted us to explore reactions of transition metal ions with small borane and carborane anions (32). This, in turn, led to a number of advances including the first closed polyhedral metallaboranes (33), the discovery of metal-induced oxidative cage fusion (34), and the synthesis of B12H16, the only neutral Bi2 hydride, via metal-promoted fusion of BeHQ (35)... 

B may exhibit relaxation times of up to several seconds in tetrahedral environments such as in [BH4] in the liquid phase. Long relaxation times also sometimes occur in ostensibly asymmetric environments when the efg at the nucleus is fortuitously close to zero. " These long relaxation times are exceptional, however, and in general a significant asymmetry of the efg at the nucleus prevails and moderately efficient quadrupolar relaxation occurs (Chapters, Sec.4.5). Relaxation times r2("B) vary typically from >ca. 10 s for reasonably symmetric environments in smaller molecules in mobile solutions to <10 s for bulkier species such as polyhedral metallaboranes with several triarylphosphine ligands attached to the metal centers. Natural linewidths therefore typically vary from a few hertz to several hundred hertz, although occasionally some sharper lines occur. 

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