The Borane-Carborane Structural Pattern

From the early structural studies carried out on boranes, it appeared that most of these compounds adopted structures based on arrangements of their boron atoms that defined fragments of icosahedra. That other triangular-faced polyhedra were important became apparent during the 1960s 141, 145, 164, 166, 212), as the structures of key materials such as the borane anions and carboranes C2B 2H were determined, 

There are three main structural types, closo, nido, and arachno. Closo structures are adopted by borane anions ( = 6 - 12), 

The same polyhedra serve as the basis for the structures of nido and arachno compounds, too, although for these boranes and carboranes the polyhedra are incomplete. Nido structures are adopted by neutral boranes BjjHw j.4, carboranes CBy 4Hn.. 3, CsBji— 

The nido-boranes BsHg, BgHio, and B10H14, showing the fundamental polyhedra. 

The positions of the hydrogen atoms in boranes and carboranes are worth noting. In doso-borane anions the n BH units are aligned 

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This is one of two articles in this volume concerned with the borane-carborane structural pattern. In the other (see Williams, this volume, p. 67) Williams has shown how the pattern reflects the coordination number preferences of the various atoms involved. The purpose of the present article is to note some bonding implications of the pattern, and to show its relevance to a wide range of other compounds, including metal clusters, metal-hydrocarbon n complexes, and various neutral or charged hydrocarbons. 

The structures of a representative cross-section of known carboranes are shown in Figures 3.2-3.6. Though many new carboranes have been synthesized and characterized since the borane/carborane structural pattern was hrst described,these have tended to consolidate rather than greatly extend the picture. A little progress has been made with the synthesis of macropoly-hedral borane clusters in which fused smaller polyhedra share edges or faces.This area need not concern us here, as it almost exclusively involves boranes and metallaboranes rather than carboranes and metallacarboranes. 

Before exploring other areas to which the borane-carborane structural and bonding pattern is relevant, it is useful to summarize the main features, as follows. 

Chapters in the book The Borane, Carborane, Carbocation Continuum are of interest Patterns of structures in boranes and carboranes, The carboranecarbocation continuum, Untangling molecular structures, and New species of boranes and carboranes. ... 

Although the structural pattern outlined in the foregoing can be rationalized at a simple quabtative level by using a molecular orbital approach to the skeletal bonding of boranes and carboranes (see Section III, B) it is useful to consider first what problems are encountered if one attempts to describe the bonding in terms of localized bonds. 

The structural patterns illustrated in Figure 3.1, and the shapes shown in column 3 for arachno species, are essentially those first suggested by Williams in 1971,2 and have stood the test of time remarkably well. However, the shapes shown should be regarded as illustrative rather than definitive. As with the nido species, alternative shapes to those shown in Figure 3.1 are possible for arachno species, with different sites left vacant on the parent deltahedron. We have already noted that many nido boranes and carboranes with eight skeletal atoms show deviations in their atom connectivities from those shown in Figure Nevertheless, apart from these particular exceptions,... 

This paper subscribes to the third viewpoint, and is based on an empirical approach that involves coordination number pattern recognition (CNPR). It is a simplistic approach, yet it apparently accommodates most if not all carborane and borane structures. For compounds that are still controversial and for compounds that have not yet been discovered or characterized, the CNPR thesis frequently predicts different structures, or at least fewer candidates, than do any of the theoretical treatments. 

Tlie structural and bonding pattern to which carboranes and boranes themselves conform reflects their formulae, and so reflects the numbers of electron pairs holding their skeletal atoms together. Featured in many inorganic... 

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