Boron hydrides electron-deficient clusters

Williams [1] has given an excellent review on Early Carboranes and Their Structural Legacy and he defines carboranes as follows Carboranes are mixed hydrides of carbon and boron in which atoms of both elements feature in the electron-deficient polyhedral molecular skeleton . According to the electron counting rules [2] for closo- (2n + 2 SE), nido- (2n + 4 SE) and arachno-clusters (2n + 6 SE SE = skeletal electrons, n = number of framework atoms) and the An + 2 n electron Hiickel rule, small compounds with skeletal carbon and boron atoms may have an electron count for carboranes and for aromatics (see Chapters 1.1.2 and 1.1.3). 

Metallacarbaborane (also called metallacarborane) an electron-deficient compound, usually a polyhedral cluster comprising an array of boron-hydride (BH), carbon-hydride (CH), and metal (ML, where L = ligand) fragments the inclusion of a substituted carbon fragment CR (R = alkyl, aryl, or trimethylsilyl) in place of a CH unit is common... 

Hydrogen bridges between the beryllium atoms produce a polymeric structure for BeH2, as shown in Fig. 18.6. The localized electron model describes this bonding by assuming that only one electron pair is available to bind each Be—H—Be cluster. This is called a three-center bond, since one electron pair is shared among three atoms. Three-center bonds have also been postulated to explain the bonding in other electron-deficient compounds (compounds where there are fewer electron pairs than bonds), such as the boron hydrides (see Section 18.5). 

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