Boron borane structure

Boranes are recognized as clusters of boron atoms which represent triangularfaced polyhedra. It was Williams62 in 1970 who pointed out in a classic paper that the structures of all boranes are derived from those of the closed polyhedral borane anions, or carboranes, with vertices ranging from 5 to 12. These regular polyhedra are shown in Fig. 5 and represent the structures of the closo-boranes (or carboranes) from which all other borane structures are derived. 

This section presents the structure of the boron hybrides and is arranged in accordance with the relationship defined by Wade s Rules and expressed by Williams and Rudolph. Thus for the boranes containing six or more pairs of skeletal bonding electrons, the relationship between the structures of the closo-, nido-, arachno-and hypho-species is described. In cases where the parent borane does not exist, examples from heteroboranes with the correctly predicted structure based on Williams coordination number pattern recognition theory (CNPR) of borane structures will be described 70. Treated separately will be mono- and diborane species and also species with more than 12 boron atoms. Although there have been several reviews on the structures of the boranes in recent years none have used the current approach89. ... 

Boron hydride structures and structures of molecules related to them, as well as other properties associated with bonding problems in these compounds, have been reviewed in the first volume of this series (83). Here it is intended to survey chemical reactivity of the boranes and of their derivatives. 

On the basis of known behavior most boron hydride reactions may be assigned to two main classes. The first class involves removal of a borane group from a boron hydride structure, while the second class involves nonsymmetrical cleavage of the hydrogen bridge bonds in a borane.2... 

M.M. (2001) Polyhedral boranes and elemental boron direct structural relations and diverse electronic requirements. J. Am. Chem. Soc., 123 (18), 4324-4330. 

Fast-forwarding to the present, I have selected a few examples of current research activity in boron cluster science that in one way or another build on the fundamental discoveries and bonding models of polyhedral borane structures that originated a half century ago with Lipscomb and his compatriots. While current publications in these areas may or may not explicitly acknowledge this original work, it is in fact the foundation upon which modem studies of polyhedral boranes and related clusters rest. 

FIGURE 22.12 nido- and arachno-Botanes The rarfo-borane structure forms a cage missing one boron atom from a comer. The amchno-hoiane stmcture forms a web missing more than one boron from a comer. 

In this case the covalency of boron is brought up to four because the donor molecule supplies the necessary electrons. The adduct formed, trimethylamine-borane, is a stable white solid. Other compounds of a similar kind are known, all derived from the simple structure H3N -> BH3. This compound is isoelectronic with ethane, i.e. it contains the same number of electrons and has the same shape. 

Boron (like silicon) invariably occurs in nature as 0X0 compounds and is never found as the element or even directly bonded to any other element than oxygen. The structural chemistry of B-O compounds is characterized by an extraordinary complexity and diversity which rivals those of the borides (p. 145) and boranes (p. 151). In addition, vast numbers of predominantly organic compounds containing B-O are known. 

Wells et al. characterized group 13-stibine adducts by single crystal X-ray structure analyses first in 1997 [35]. The solid state structures of three borane-stibine adducts of the type X3B—Sb(Tms)3 (X = Cl 6, Br 7, I 8), obtained by reaction of boron trihalides BX3 and Sb(Tms)3 in n-pentane, were determined. 

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