Bonding carboranes

There is mass spectroscopic evidence for clusters C59B, C68B2,. ..C54 B6 in the products of laser ablation of boron nitride/graphite composites (Guo et al. 1991). It is proposed that these may be versions of the fullerene cage in which heteroatoms have occupied carbon sites with little disturbance of the framework. This is a plausible hypothesis for low doping ratios, in view of the ability of carbon to substitute for boron in carboranes. Bond-energy considerations suggest that... 

The main geometries encountered in this review are shown in Fig. 1. In the closed (closo) structure (Fig. lb) the skeletal atoms occupy all corners of a polyhedron, while in the open (nido) structure (Fig. la) one corner of the polyhedron is vacant. This open face is usually the location of attachment when the carborane bonds to a metal group. In some cases, the metal can occupy common vertices of two polyhedra to give a commo structure (Fig. lc). 

The reaction of (fluoroalkenyl)carboranes with potassium permanganate in acetone leads to formation of a-diketones [119. These compounds react by photochemical reaction in which the radical formation at boron is followed by addition to the double bond [120] (equation 92). 

C = 12 internationally adopted as the unified atomic weight standard by both chemi.sts and physicists. 6-coordinate carbon established in various carboranes by W. N. Lipscomb and others. (Nobel Prize 1976 for structure and bonding of boranes). 

Some group-IB and -IIB boranes or carboranes, especially of Cu, possess M — H—B structure units and are not discussed here because no direct metal-boron bond exists. 

The formation of boron-group IB bonds succeeds in two ways by transfer of a boryl group from metal-boron compounds to other metals, and by reaction of anionic boranes or carboranes with transition-metal halides. 

The structure of the disilanol l,7-bis[hydroxy(dimethyl)silylmethyl]-m-carborane has a hydrogen-bonded network reminiscent of the sil-oxanediol type shown in Fig. 10c, in which pairs of molecules are hydrogen-bonded together and then these pairs are further linked to give a double chain (290). 

The c/two-boranes BnH - (5 < n < 12) and the carboranes Bf C2Hf +2 are showpieces for the mentioned Wade rule. Further examples include the B12 icosahedra in elemental boron (Fig. 11.16) and certain borides such as CaB6. In CaB6, B6 octahedra are linked with each other via normal 2c2e bonds (Fig. 13.13). Six electrons per octahedron are required for these bonds together with the 2n + 2 = 14 electrons for the octahedron skeleton this adds up to a total of 20 valence electrons. The boron atoms supply 3 x 6 = 18 of them, and calcium the remaining two. 

But the discovery of the carboranes in the early 1960s revealed that bonding possibilities other than simple o-- or 7r-bonds between B and C centers were necessary to understand the structure of such compounds as 1,5-dicarba-closo-pentaborane(5) [6 in Eq. (1)], which is obtained in low yield in an electrical discharge.6 Ordinary valence conventions cannot account for the bonding of boron to five other atoms, and hence the concept of electron-deficient bonding must be invoked for boron. Although carbon seems to adhere to normal tetravalence, again it should be remembered... 

This review will restrict itself to boron-carbon multiple bonding in carbon-rich systems, as encountered in organic chemistry, and leave the clusters of carboranes rich in boron to the proper purview of the inorganic chemist. Insofar as such three-dimensional clusters are considered at all in these review, interest will focus on the carbon-rich carboranes and the effect of ring size and substituents, both on boron and carbon, in determining the point of equilibrium between the cyclic organoborane and the isomeric carborane cluster. A typical significant example would be the potential interconversion of the l,4-dibora-2,5-cyclohexadiene system (7) and the 2,3,4,5-tetracarbahexaborane(6) system (8) as a function of substituents R (Eq. 2). 

Carboranes also offer the capacity for Co111—C bond formation. Perhaps the best-known example of a mixed cyclopentadienyl-metallocarborane sandwich complex is the cobalt(III) compound (Cp)Co(l,2-C2B9Hn), which serves as a precursor for other species.544... 

Monoanionic carborane anions based around [c/ow-l-CBnHi2]- are among the most inert and least coordinating anions currently known. The high chemical stability and low nucleophilicity allows the stabilization of cationic species not isolable with other anions. The silver salt of this compound was crystallized as Ag(CB11H12) 2C6H6 (236), in which the silver is bonded to two... 

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