Boranes, polyhedral

The geometries of metal carbonyl and metallocarborane cluster compounds have been systematized recently by a set of simple rules described collectively as the polyhedral skeletal electron pair theory (153, 218, 232). This approach originated from a perceptive analogy between isostructural metal carbonyl and borane polyhedral cluster molecules (232), and its applications have been widely discussed and reviewed (147, 153, 210, 218, 233, 234, 235, 240). In this review,... 

Octahydrotriborate (1 — )ion, [B3H8], is an important intermediate in the synthesis of higher boranes, polyhedral borane anions, and transition-metal complexes. Salts containing this ion have usually been prepared1 from diborane, which is toxic, spontaneously flammable, and expensive. In addition, pressure facilities are often required for the synthesis. 

TiTetal complexes containing borane or borane anion ligands have often been referred to as metalloboranes, especially when the metal can be considered as occupying a boron position in a borane polyhedral fragment. Outlined here are some of our recent experimental findings in the area of metalloborane chemistry. Much of the earlier work was reviewed elsewhere 1,2, 3),... 

There are boron hydrides in which one or more of the polyhedral sites is truly removed. Figure 3-26b shows the systematics of borane polyhedral fragments as obtained from closo boranes, after Williams [3-27] and Rudolph [3-24]. All the faces of the polyhedral skeletons are triangular, and thus the polyhedra may be termed deltahedra and the derived fragments deltahedral. The starting deltahedra are the tetrahedron, the trigonal bipyramid, the octa-... 

Polyhedral Expansion. The term polyhedral expansion is used to describe a host of reactions in which the size of the polyhedron is increased by the addition of new vertex atoms whether boron, heteroelements, or metals. In the case of the boranes, the pyrolysis of B2H has been used to obtain B H and industrially. Although a subject of much study, the mechanism of such pyrolytic expansions is not well understood. 

Closo Borane Anions. This group contains a homologous series of very stable polyhedral anions, [doso —, n = 6-12. Just as the... 

Pyrolysis of CsjB Hg] at 230°C gives CS2IB2H2] (60%) along with some CS2IB2QH2Q], CS2IB22H22], and CsBH (93). The sensitivity of polyhedral expansion reactions to solvent, temperature, and pressure is further exemplified by the results ia dioxane at 120°C under pressure. To obtain the closo borane, NajB H J is first converted to Cs2[B2 H23], which can be pyrolyzed to give Cs2[B2 H2J (89). 

Boranes also form derivatives ia which main group elements occupy a bridging position between two boron atoms, rather than a polyhedral vertex. An extensively studied system is -R MB Hg, where R = H, CH, C2H3, halogen, and M = Si, Ge, Sn, Pb (185). The stmcture of l-Br- J.-[(CH3)3Si]-B3H2... 

In most cases the prolonged treatment of a closo carborane or borane with strong base results in the removal of a single-boron vertex to yield a nido cluster, inert to further degradation. This principle is exploited in the polyhedral contraction and subrogation synthetic strategies. In the prototypical case. 

Cluster-expansion and cluster-degradation reactions are a feature of many polyhedral borane species. Examples of cluster-expansion are " ... 

Decaborane is the most studied of all the polyhedral boranes and at one time (mid-1950s) was manufactured on a multitonne scale in the USA as a potential high-energy fuel. It is now obtainable in research quantities by the pyrolysis of B2H9 at 100-200°C in the presence of catalytic amounts of Lewis bases such as Me20. B10H14 is a colourless, volatile, crystalline solid (see Table 6.2, p. 163) which... 

As a result of the systematic application of coordination-chemistry principles, dozens of previously unsuspected stnicture types have been synthesized in which polyhedral boranes or their anions can be considered to act as ligands which donate electron density to metal centres, thereby forming novel metallaboranc elusters, ". Some 40 metals have been found to act as acceptors in this way (see also p. 178). The ideas have been particularly helpful m emphasizing the close interconnection between several previously separated branches of chemistry, notably boron hydride clu.ster chemistry, metallaboranc and metallacarbaborane chemistry (pp. 189-95). organometallic chemistry and metal-metal cluster chemistry. All are now seen to be parts of a coherent whole. 

E. L. Meutterties and W. H. Knoth, Polyhedral Boranes, Marcel Dekker, New York, 1968, 197 pp. 

The chemistry of polyhedral borane ions. L. J. Todd, Prog. Boron Chem., 1970,2,1-35 (81). 

The discovery of polyhedral boranes and polyhedral heteroboranes, which contain at least one atom other than in the cage, initiated a new era in boron chemistry.1-4 Most commonly, of the three commercially available isomeric dicarba-closo-dodecaborane carboranes(l,2-, 1,7-, and 1,12-), the 1,2-isomer 1 has been used for functionalization and connection to organic molecules. The highly delocalized three-dimensional cage bonding that characterizes these carboranes provides extensive thermal and kinetic stabilization as well as photochemical stability in the ultraviolet and visible regions. The unusual icosahedral geometry of these species provides precise directional control of all exopolyhedral bonds. 

L Monomeric and Polymeric Organic Analogs of Boron Cluster Systems The polyhedral clusters of boranes and carboranes are groups of cluster systems that are present ubiquitously in organoboron polymers. As in the case with the... 

The area of chemistry involving the polyhedral boranes and carboranes has seen enormous growth in recent years. Accordingly, the brief survey here does not include many interesting facets of these fascinating compounds. For more details on the polyhedral boranes, the references at the end of this chapter should be consulted. 

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