One of a class of binary compounds of boron and hydrogen, often referred to as boron hydrides. The term borane is sometimes used to denote substances which may be considered to be derivatives of the boron-hydrogen compounds, such as boron trichloride (BCl3), and diiododecaborane (B10H12I2).
The simplest borane is diborane (B2H6); other boranes of increasingly higher molec­ular weight are known, one of the least volatile of which is an apparently polymeric solid of composition (BH)x. Certain boranes, such as BH3, and B3H7, are not known as such, but can be prepared in the form of adducts with electron-donor molecules.
The most spectacular projected large-scale use of the boranes and their derivatives is in the field of high-energy fuels for jet planes and rockets. The thermal decomposition of diborane (6) [B2H6] has been used to produce coatings of pure elementary boron for neutron-detecting devices and for applications requiring hard, corrosion-resistant surfaces. Boranes can also be used as vulcanizing agents for natural and synthetic rubbers, and are especially effective in the preparation of silicone rubbers.
The molecular structures possessed by the boranes are exhibited by no other class of substances. Because of the lack of sufficient electrons for the formation of the requisite number of covalent bonds, normal covalently bonded structures of the hydrocarbon type are not possible. The boranes are sometimes referred to as electron-deficient substances. In no case are the simple chain and ring configurations of carbon chemistry encountered in the more complex boranes. Instead, the boron atoms are situated at the comers of polyhedrons. An example of such a structure is that of pentaborane (9) [B5H9]. Boron nomenclature uses a prefix to designate the number of boron atoms in the molecule and a numeral suffix in parentheses to indicate the number of hydrogen atoms.
As a class, the boranes are quite reactive substances and are generally decomposed, at times explosively, on contact with air. Their reactivities with air and water decrease with increasing molecular weight. Because boranes react readily with air, laboratory investigations are almost invariably carried out in all-glass vacuum apparatus or in inert-atmosphere dry boxes. With the possible exception of decaborane (14) [B10H14], the known boranes are not indefinitely stable at room temperature. They decompose more or less rapidly to yield elementary hydrogen and boranes richer in boron. The known derivatives of the boranes ( other than BH3) are relatively few in number. Several halo, alkyl, and amino boranes have been reported but, in general, these have not been extensively characterized.