Borane chemistry structures

Even though qualitative bonding descriptions of metal atom clusters up to six or seven atoms can be derived and in some cases correlated with structural detail, it is clear that most structures observed for higher clusters cannot be treated thus. Nor do the structures observed correlate with those observed for borane derivatives with the same number of vertices. Much of borane chemistry is dominated by the tendency to form structures derived from the icosahedron found in elemental boron. However, elemental transition metals possess either a close-packed or body-centered cubic arrangement. In this connection, one can find the vast majority of metal polyhedra in carbonyl cluster compounds within close-packed geometries, particularly hexagonal close-packing. 

An unusual type of deltahedral cluster found in organoindium chemistry but not in deltahedral borane chemistry is the eight-vertex In8[Si(CMe3)3]6 (see Chapter 2.3.4.2) its structure is based on a bicapped octahedron with idealized D34 sym-... 

The foregoing examples show the relevance to metal-carbonyl cluster chemistry of the borane-oarborane structural and bonding pattern. Its relevance to other areas of chemistry may be explored readily using a systematic skeletal electron-counting procedure (161, 201). 

Localized Bonds. Because boron hydrides have more valence orbitals than valence electrons, they have often been called electron-deficient molecules. This electron deficiency is pardy responsible for the great interest surrounding borane chemistry and molecular structure. The structure of even the simplest boron hydride, diborane(6) [19287-45-7], B2H , was sufficiendy challenging that it was debated for years before finally being resolved (57) in favor of the hydrogen bridged structure shown. 

A question of interest in deltahedral borane chemistry is whether supraicosahedral boranes B H 2 (n > 13) can be prepared and whether the Frank-Kasper polyhedra (Figure 1) are suitable models for their structures. The supraicosahedral metallacarborane (C5H5)2Co2B10C2H,2 is known21 and has a structure based on the Frank-Kasper bicapped... 

Metal triangles, tetrahedra, and octahedra form the basic building blocks of transition metal carbonyl clusters. The smaller clusters with between three and six metal atoms often adopt these pseudo-spherical deltahedral geometries, but as the nucle-arity of the cluster increases, condensed structures, built up from the smaller poly-hedra by vertex-, edge-, or face-sharing, tend to be favored in preference to the larger spherical deltahedra based on the Platonic and Archimedean solids. In general, this is in contrast to the structures foimd in borane chemistry. 

The impact of the studies of William N. Lipscomb and his collaborators on the development of polyhedral borane chemistry, and more broadly on the general field of polyhedral clusters, is reviewed. After a brief recapitulation of the state of knowledge of borane structures and bonding at the start of Lipscomb s career in the 1940s, the influence - direct and indirect - of his work on the shaping of this area, and on the development of cluster science into its current multifaceted form, is explored with the aid of selected examples. The role of scientific imagination in chemistry, as exemplified by minds as diverse as those of Jefferson, Lewis Carroll, and Lipscomb, is highlighted. 

Carboranes were first reported in 1962. Much of the early work was done in the USA as part of a classified project on borane chemistry, seeking novel rocket fuels, an object which proved abortive. Since then thousands of compounds of this type have been reported. Carboranes have structures which are related to those of the boranes by isoelectronic replacement of groups (e.g. B" or BH by C BH or BH by CH). They may belong to closo, nido or arachno series, although the last are relatively uncommon. The usual entry to carborane chemistry is to heat a borane with an alkyne or to subject the mixture to a silent electric discharge. Thus nido-BjH, and ethyne at 200°C give mainly nido-I S-C BJl, while at 500°C a... 

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