Carborane polyhedra

Our discussion will be restricted to metallacarboranes in which the r/-block metal is incorporated into the polyhedral framework of the carborane. No attempt will be made to cover those compounds where the transition metal atom is in a bridging group linking several carborane polyhedra together or is involved solely as a member of a substituent group. Since much of the insight into the chemistry of the metallacarboranes is obtained from structural and bonding... 

Isomerization reactions, whereby adjacent carbon atoms in carborane polyhedra move apart, allow more stable isomers to be prepared from the less stable by heating [Eqs. (3.7) and (3.8)] ... 

Poly(carborane-siloxanes) have a linear structure in which R and R" can be alkyl, fluoroalkyl or aryl groups. The main chain contains the carborane polyhedra. Most commonly C2B10H10 is the carborane used. 

Among group 2 elements, only beryllium is known to have been inserted into a carborane polyhedron. Even though cyclopentadienyl rr complexes of the heavier alkaline earth meals are known, analogous carborane complexes have not been reported. 

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. 

Although most known metallocarboranes have only 1 metal atom per polyhedron, the existence of an increasing number of metallocarboranes with 2 or more metal atoms per cluster emphasizes the close relationship between metal clusters and borane clusters. Since they can be synthesized from cZoao-carboranes by replacing BH units by metal-carbonyl or metal-cyclopentadienyl residues, carborane clusters can effectively be used as disposable templates on which to fabricate... 

One particularly interesting category of metallocarborane is that in which a single metal atom is shared between two polyhedra that have a vertex in eommon. In effect, the metal is sandwiched between two nido-carborane residues. Examples are shown in Fig. 17. For such commo compounds, the metal can be assumed to contribute three AO s to the skeletal bonding of each polyhedron, when the (n + 1) rule for closo clusters is found to be obeyed. For example, the isoelectronic... 

Spencer, Green, and Stone (88) have recently described a new synthetic approach to metallocarboranes in which an organomctallic transition metal complex is thermally reacted with a neutral cZoso-carborane, resulting in the incorporation of the metal into the polyhedron ... 

Boranes of the general formula B Hn2 and isoelectronic carboranes such as CB iHk- and C2B 2H have closo structures, in which n skeletal B and C atoms are located at the vertices of a polyhedron bounded by trianglular faces (deltahedron). For the BMH 2 cfo.vo-boranes... 

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). 

It is not necessary to consider the icosahedron as the largest c/oro-polyhedron from which other boranes are derived. Lipscomb55 has described hypothetical c/oro-boranes [BnHJ2- where n = 13 to 24. Furthermore, several examples of metallo-boranes and -carboranes are known. Callahan and Hawthorne152 have described several c/oso-metallocarboranes containing 13 and 14 vertices and Maxwell and... 

We have shown for the first time 12 that not only c/oso-carboranes, but also iodo-nido-7,8-dicarbaundecaborates containing an iodine atom in the polyhedral skeleton or in the pentagonal plane of the polyhedron enter into cross-coupling reactions with organomagnesium and organozinc compounds catalyzed by palladium complexes. It is known that ifo-7,8-dicarbaundecaborates are strong BH-acids (pKa = 21.1), due to the presence of the axial hydrogen atom. 

Twelve Vertices. Twelve-atom cages are not widespread but play a dominant role in boron chemistry. The most highly symmetrical arrangement is the icosahedron [Fig. l-8(b)], which has 12 equivalent vertices and Ih symmetry. Icosahedra of boron atoms occur in all forms of elemental boron, in B12Hi 2 and in the numerous carboranes of the B10C2H12 type. A related polyhedron, the cuboctahedron (1-LI) is found in several borides of stoichiometry MB12. As shown on page 249, the icosahedron and cuboctahedron have a rather close relationship. 

Boranes and carboranes have structures in which their skeletal B- or C-atoms form triangular-faced polyhedra. There are basically three structural types, namely the closo- (an euphonious modification of the Greek clovo = cage, i. e., a complete or closed polyhedron), the nido (from Latin nest-like ) and the arachno- (from Greek cob-web ) structure. Each of these three types is adopted by cluster compounds of specific atomic ratios. c/o o-Structures occur in borane dianions B H , in car-borane anions (CB iH ) , and carboranes (C2B 2H ). Each skeletal atom has a single H-atom terminally attached by a bond directed outwards, away from the polyhedron center (see the example of BioHio in Fig. 3-1 below). Wo-Structures are adopted by boranes B H +4 and their related carboranes CB iH +3, C2H 2H +2 etc., and amc/z/2o-structures by boranes B H +6 and related carboranes CB iH +5, C2B 2H +4 etc. In other words, carboranes have the general formula [(CH) (BH) Hc] , where the sum a + c + x) is equal to 2 for a closo-structure, 4 for a /do-structure, and 6 for an amc/z o-structure. 

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