Structures and bonding of some higher boranes and boron polyhedra

Abstract

We saw in Chapter 2 how the bridge bonding in diborane could be described simply in terms of two banana-shaped three-centre electron-pair bonds, although an understanding of the overall distribution of the valence electrons of diborane required a molecular orbital treatment which took into account the symmetry of the whole molecule. The concept of three-centre electron-pair bonds can profitably be applied to a wide variety of other electron deficient compounds. W. N. Lipscomb,1 for example, has shown how its use allows a relatively simple bonding interpretation to be made of the seemingly complicated structures of most of the higher hydrides of boron, as outlined below. Not all electron deficient compounds of boron have structures that are readily interpreted in terms of simple three-centre bonded schemes, however, and the second half of this chapter is concerned with some cage structures that figure in the hydride, halide, and alloy chemistry of boron as well as in the element itself, structures in which boron atoms are located at the corners of the equilateral triangular faces of polyhedra containing no bridging hydrogens. Bonding schemes for such structures are best arrived at by more or less full molecular orbital treatments.