Boron hydride bridged systems

The three-centered, two-electron hydride bridge, which is prevalent in boron hydride chemistry, has been very well characterized in the ir and Raman spectroscopy of diborane and certain metal borohydrides. A brief review of these data will be given at this point because they afford insight into hydride-bridged systems. 

Concerning the inherent donor character of the ligand, this point simply considers the possibility that while the inductive effect might favor unsymmetrical cleavage, the base L could be a weak donor to boron compared to hydride. Thus formation of the symmetrical cleavage product could occur after initial unsymmetrical cleavage of the bridge system. 

Boron hydrides thus contain four different kinds of bonds ordinary boron-hydrogen bonds, boron-hydrogen-boron three-center bridge bonds, ordinary boron-boron bonds, and boron-boron-boron three-center bonds. W. N. Lipscomb developed a method for determining what combinations of these structural features are possible for a specific boron hydride formula and what are the possible structures. This system is equivalent to the simple bonding rules in organic chemistry that make it possible to translate an empirical formula into possible organic structures. 

After removing a hydride of the cycloheptatriene and a proton of cyclopentadiene the bis-substituted p-carborane can be compared with the analogous organic [5.6.7]quinarene derivative shown in Figure 13.14 (left side), but there is one major difference between than the bridging benzene unit allows electronic communication between the tropylium and the cyclopentadienide unit and thus a fully electron-delocalized system exists, while the boron-based polyhedral bridge restricts the electron transport, which leads to a highly polarized molecule (Taylor et al., 2001). 

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