Carboranes structure

R. E. Williams, Coordination number-pattern recognition theory of carborane structures, Adv. Incrg. Chem. Radiochem. 18, 67-142 (1976). R. E. Williams, Chap. 2 in G. A. Olah, K. Wade and R. E. Williams (eds.). Electron Deficient Boron and Carbon Clusters, Wiley, New York, 1991, pp. 11-93. 

Coordination Number Pattern Recognition Theory of Carborane Structures Robert E. Williams... 

See also Carboranes structural systematics, 4 172-179 weakest anions and strongest acids, 4 202-204... 

Ab initio computations, particularly at MP2 and (the computationally less intensive) B3LYP levels of theory, have been largely responsible for the rapid progress in carborane structural research, permitting the determination of the molecular geometry of a novel carborane from solution-state NMR data where X-ray diffraction could not be used (such as those in trace amounts or for which suitable crystals could not be obtained). Earlier... 

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. 

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. 

COORDINATION NUMBER PATTERN RECOGNITION THEORY OF CARBORANE STRUCTURES ... 

Several schools of thought then arose. Members of one school treated each structure as a separate case, more or less succumbing to the thesis of an almost infinite variety of structural parameters. A second school took the view that although carborane structures were complex in nature, such structures could eventually be categorized or collated by improved theoretical treatments which could be expected to become ever more accurate because of constantly improving computer systems. A third school considered the complexity of carborane structures to be not as severe as had appeared at first view, arguing that when a sufficient number of structures would eventually be determined, the fundamental structural precepts would become decipherable and, consequently, amenable to a simplistic empirical organizational format. 

Included in the 1971 reappraisal 164) of carborane structures, and contrary to many alternatively suggested structures pervading the literature at that time, were the following predictions ... 

Almost all of the ido-carboranes have bridge hydrogens, and the organization of their structures is much more complicated than closo-carborane structures because the bridge hydrogens (rule 2) apparently... 

Identical considerations suggest that the stabilities of the three isomers of CSB7H11 will decrease in the order XIII- > XIV- > XV-NlO. So also the C4BeHxo isomers should be less stable in the order XVII- 20, 165), XVTII-, XIX-NIO 14). Brown et al. 14a), at the University of Liverpool, have shown unequivocally that H4C4B0(CH3)e has an adamantane-type structure with sp boron rather than the carborane structure with sp boron. 

On the preceding pages there has been an attempt to present carborane structural chemistry as clear and orderly. In fact, however, many bits of information are missing, perhaps have been misplaced, or misinterpreted additional research is needed to fill in the blanks and to correct the errors. 

Figure 5-13 A selection of nido- and arachno-carborane structures, (a) nido- 1,2-C2B3H7. (b) nido-2-CBsH9. (c) nido-2,3-QB4H8. (d) irfo-2,3,4-C3BjH7. (e) nido-2,3,4 -C U. (f) arachno-C n. (g) nido-QB9H13. (h) arachno-C3B7H13. (i) nido-CABoH12. Compounds (h) and (i) are known only as C-substituted derivatives.

Some carboranes have formulae compatible with both classically bonded and nonclassically bonded structures and exhibit valence isomerism. The tet-racarbaborane Me4C4B6Et6 is one such. As prepared by dimerization of the small closo carborane Me2C2B3Et3 [Eq. (3.2)], it has a classical adamantane-type structure, with its four CMe units linked through BEt units (1). However, when heated it isomerizes to the expected nido carborane structure (2) with a skeletal structure like that of decaborane. ... 

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. 

PINSA-A, Proceedings of the Indian National Science Academy, Part A Physical Sciences, Vol. 68, Indian National Science Academy, 2002 R 276 R.E. Williams, A Dozen Carborane Structures Refined by Comparison of Quantum Chemical Calculated and Experimental NMR Chemical Shift Values , p. 499... 

---