Boron hydrides atomic carbon

This chapter concerns carboranes (carbaboranes), which are boron clusters with at least one carbon atom as part of the polyhedral cage. Published studies on carboranes before 1981 were reviewed in GOMC (1982) and between 1982 and 1992 in COMC (1995). The present review covers the period of 1992 to early 2005. Unlike in previous chapters, boron hydrides with organic substituents attached to a boron atom, organopolyboron hydrides, are not discussed in this chapter. Borane clusters containing at least one non-carbon atom as part of the cage framework are reviewed in Chapters 3.03, 3.04 and 3.05 of this volume. 

Williams [1] has given an excellent review on Early Carboranes and Their Structural Legacy and he defines carboranes as follows Carboranes are mixed hydrides of carbon and boron in which atoms of both elements feature in the electron-deficient polyhedral molecular skeleton . According to the electron counting rules [2] for closo- (2n + 2 SE), nido- (2n + 4 SE) and arachno-clusters (2n + 6 SE SE = skeletal electrons, n = number of framework atoms) and the An + 2 n electron Hiickel rule, small compounds with skeletal carbon and boron atoms may have an electron count for carboranes and for aromatics (see Chapters 1.1.2 and 1.1.3). 

Nucleophiles such as hydroxide or cyanide ion, in the presence of an oxidant, cause deboronation of ions (99) and (100) to give ion (96) and benzeneboronic acid. If no other oxidant is present the ions (99) and (100) themselves serve as oxidizing species to give a disproportionation. The use of mild reaction conditions stops the oxidation of (100) at the stage of (99). Nucleophiles like pyridine and hydride ion (from sodium borohydride) add to the borinate ring, pyridine to the boron atom and hydride to carbon atoms (79CB607). 

CARBORANE. A cry stalline compound composed of boron, carbon, and hydrogen. It can be synthesized in various ways, chiefly by the reaction of a borane (penta-or deca-) with acetylene, either at high temperature in the gas phase or in the presence of a Lewis base. Alkylated derivatives have been prepared. Carborancs have different structural and chemical characteristics and should not be conTused with hydrocarbon derivatives or boron hydrides. The predominant structures arc the cage type, the nest type, and the web type, these terms being descriptive of the arrangement of atoms in the crystals. Active research on cargorane chemistry has been conducted under sponsorship of the U.S. Office of Naval Research, http //www.onr.navy.mil/... 

In the boron hydrides, called boranes, hydrogen often serves as a bridge between boron atoms, a function rarely performed by hydrogen in carbon chemistry. How is it possible for hydrogen to serve as a bridge One way to address this question is to consider the bonding in diborane, B2H6 ... 

The carboranes are mixed hydrides of carbon and boron having both carbon and boron atoms in electron deficient skeletal framework. 

A zwitterionic [RNHj]" [CB,oH,2] obtained from the action of alkylisocyanides on decaborane(14) is a cage insertion of one carbon atom into a borone hydride framework to give another compound (as a nitrogen derivative) in the nido-carborane series, CB H +4 ... 

The compound borane-carbonyl occupies a special position in any discussion of molecules derived from boron hydrides with boron-carbon bonds. Carbon monoxide reacts with diborane (20 atm/room temperature) to give an adduct of the borane group which does not rearrange or lose hydrogen (39) The adduct OC BH3 does, however, dissociate into carbon monoxide and diborane at ordinary temperatures. No carbonyls of other Group III acceptor molecules have been prepared so far 133), and it has been suggested 27, 58) that borane-carbonyl owes its existence to the ability of the hydrogen atoms of borane to transfer electrons to a vacant p,-orbital in carbon monoxide. 

Inorganic complexity does not require the carbon atom at aU in some cases. The German chemist Alfred Stock (1876-1946) began the study of boron hydrides (compounds of boron and hydrogen) in 1909. He found that fairly complicated compounds could be built up, compounds analogous, in some ways, to the hydrocarbons. 

In boron and beryllium there are fewer electrons than available orbitals. The outer orbitals can be filled, for example, in dimethyl beryllium, by a mechanism whereby one carbon atom orbital overlaps with an orbital from each of two neighboring beryllium atoms. The resulting high-molecular-weight chain of dimethyl beryllium thus has three center bonds, which leads to an absurd structural formula when valences are represented in the normal way. Boron hydrides and some aluminum compounds have similar structures ... 

Carborane plastics are produced from the boron molecule (a boron hydride of composition B10FI14) by replacing the four terminal hydrogens with two carbon atoms. The monomer polymerizes at high heat and pressure to form rubbery solids, which will withstand temperatures above 315°C but the commercial plastics are usually copolymers with vinyls, sihcones, or other plastics. Carboranes have been used as specialty adhesives in the aerospace industry. 

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