Cluster boron-carbon

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). 

The formation of boron-carbon bonds between the boron atoms of the clusters and carbon atoms of substituents has been described [51, 52]. These reactions have, however, not been used for the preparation of compounds useful for BNCT. Rather, the connection between the cluster and organic moieties via heteroatoms such as S, O, and N, has been achieved, and several compounds of interest to BNCT have been prepared by subsequent reactions on these heteroatoms. 

Wade electron counting rules borane-like cluster nomenclature. On initially studying compounds such as boranes (boron hydrides) and carboranes (or carbaboranes boron—carbon hydrides), Wade (1976) proposed a number of rules which have then been extended to several compounds and which relate the number of skeletal electrons with the structure of deltahedral clusters. A polyhedron which has only A-shaped, that is triangular, faces is also called a deltahedron. 

The recent work of Grimes and his co-workers 14, 26, 84, 96-96b, 98, 119, 191, 192) has shown the smaller carboranes to be particularly fertile sources of metal-boron-carbon clusters, of which a few further examples are shown in Fig. 19. 

Details of the preliminary lUPAC nomenclature for metallacarbaborane clusters are described in the 1990 recommendations. However, care is needed in interpreting compound names in the literature. Over the years, there has been some variation with respect to the order in which boron, carbon, and metal atoms are numbered in a given cluster. An example is (4), which is described as 3-Cp-3-Fe-l,2-C2B10H12 by the lUPAC 1990 recommendations, but is also found in the literature as 3-(CpFe)-l,2-C2BioHi2, 1-Cp-l-Fe-2,3-C2BioHi2, or l-(CpFe)-2,3-C2BioHi2. 

K-bonding electrons to form the metal-carbon bonds their structures may be regarded as mixed metal-carbon clusters, the shapes of which clearly reflect the numbers of electrons available as do the mixed boron-carbon cluster shapes of carboranes. All are members of the same family of hypercarbon systems. 

Many metal carbonyl clusters have interstitial atoms or groups located in the eenter of the polyhedron. Such interstitial atoms may be a light atom sueh as boron, carbon, or nitrogen a post-transition element such as germanium, tin, or antimony or a transition metal. Interstitial atoms most frequently provide all of their valence electrons as skeletal electrons since all of their valence orbitals are neeessarily internal orbitals because of the location of the interstitial atom in the center of the polyhedron. Exceptions to this rule may occur when some of the valence electrons of the interstitial atom occupy orbitals of symmetries which cannot mix with any of the molecular orbitals arising from the polyhedral skeletal bonding. 

Why do boron and transition metal hydrides tend to form clusters, when carbon and sulfur hydrides tend to form open-chain hydrides Me(CH2) Me, and HS(S) SH Why is sulfur able to form clusters in the compounds mentioned in question 2 ... 

NANOSIZED CLUSTERS or nanoclusters is a term given to particles of any kind of matter, the size of which is greater than that of typical molecules, but is too small to exhibit characteristic bulk properties. Examples of such materials include the family of carbon-based fullerenes, weakly bound van der Waals clusters observed in the gas phase, metal carbonyl clusters, boron carbides and hydrides, metal clusters surrounded by protecting ligands, and metal ions connected by bridging ligands. Of special... 

P(CH3)3. UV-photoelectron spectroscopy and Fenske-Hall molecular orbital calculations have been used to compare the species and its derivatives with the analogous cluster (p-H)2(CO)9-OS3CCO and its derivatives. The conclusions of the study are that when bound to a metal cluster, boron can act as pseudometal atom. Apparently the orbitals of the apical boron atom are less stable than those of an apical carbon atom and thus are in a better position to interact with high-lying metal orbitals [12]. 

Electron Defic. Boron Carbon Clusters, 1991,215 (CAN 228987) MRegilz, ACIE 674 P.Boudjouk, PO 1231. 

Electron-Deficient Boron and Carbon Clusters (ed. with Wade and Williams), 1991. 

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. 

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