Carboranes classical structures

Multicenter bonding is the key to understanding carboranes. Classical multicenter n bonding gives rise to electron-precise structures characteristic of Hiickel aromatics, which are planar and have 4n + 2 n electrons. Clusters are defined here as ensembles of atoms connected by non-classical multicenter bonding , i.e., all... 

One of the differences between nonclassical and classical structures (as defined above) was the existence of vacancies on boron atoms (i.e., a trivalent boron with an empty or quasi-empty p-orbital). This property makes the clas.sical structure a better Lewis acid than the nonclassical structure. Thus, when a Lewis base coordinates to a boron hydride or carborane, often the classical structure is obtained. Table 6 indicates some of the boron hydrides and carboranes for which an adduct is known. 

The doso-carboranes with a C2 B3 skeleton are of particular interest because their structure and bonding have been described as classical and non-classical. Koster et al. [49] reported the preparation of pentaalkyl-1,5-dicarba-doso-pentaboranes by hydroboration of dialkyl(l-alkynyl)boranes with tetraalkyl-diboranes(6) (R2BH)2. By using a large excess of (Et2BH)2 as a hydride bath the route to pentaethyl-1,5 -d ic ar ba- doso-pe n tabo ra n e(5) was found to proceed via the 1-carba-aradwo-pentaborane(lO) [50] (see also Section 3.2.2.4 [33]). 

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. 

Thus, in carboranes, there are fewer electrons than would be required for a classical Lewis structure in which the bonding can be described in terms of two-electron two-center bonds. Therefore, some type of multicenter bonding, such as shown in Figure 1, for diborane is required. 

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. ... 

One particularly interesting facet of carborane structures is their hybrid character, which is most evident in the more carbon-rich species such as those shown in Chart 9 (62-64). Given their intermediate position between the classical hydrocarbons and the polyhedral boranes, these compounds can be expected to (and do) reflect structural characteristics of both classes (23) and are of special interest in theoretical studies of covalent bonding. Here again, Lipscomb s contributions over several decades have markedly influenced the development of... 

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