Electron-deficient borane and carbaborane clusters an introduction

Cl ions. However, the B—B distances between adjacent Bg-octahedra are similar to those within each unit and so a discrete ion model is not appropriate. The structure type of MBi2 (e.g. UB12) can be described in terms of an NaCl stmcture in which the Cl ions are replaced by Bj2-icosahedra (Table 13.3), but again, an ionic model is not appropriate. 

Although this summary of metal borides is brief, it illustrates the complexity of structures frequently encountered in the chemistry of boron. Research interest in metal borides has been stimulated since 2001 by the discovery that MgB2 is a superconductor with a critical temperature, T., of 39 (see Section 28.4). 

11 Electron-deficient borane and carbaborane clusters an introduction 

This section introduces electron-deficient clusters containing boron, focusing on the small clusters [BgHg], B5H9 and B4HJ0. A comprehensive treatment of borane and carbaborane clusters is beyond the scope of this book, but more detailed accounts can be found in the references cited at the end of the chapter. 

An electron-deficient species possesses fewer valence electrons than are required for a localized bonding scheme. In a cluster, the atoms form a cage-like structure. 

At one time, there was considerable interest in the possibility of using boron hydrides as high-energy fuels, but in practice, it is difficult to ensure complete combustion to B2O3, and involatile polymers tend to block exhaust ducts. Although interest in fuel applications has faded, boranes remain a fascination to structural and theoretical chemists. 

A deltahedron is a polyhedron that possesses only triangular faces, e.g. an octahedron. 

In a cluster, the atoms form a cage-like structure. 

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