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compounds of boron and hydrogen. Boranes containing two to 20 atoms of boron per molecule are known. The simplest borane, BH3, does not exist in a free state; it is known only in the form of complexes with amines, ethers, and so on. The nature of borane bonds is very distinctive: they are electron-deficient compounds—that is, they do not have a sufficient number of electrons to bind all the atoms by means of the usual two-electron valent bonds. In diborane, B2H6, each atom of boron is bound to two atoms of hydrogen by ordinary bonds, but the two boron atoms are bonded by three-center “bridge” bonds, which are formed by the sharing of a pair of electrons by three atoms—two borons and one hydrogen.
The structure of other boranes is still more complex. In contrast to hydrocarbons, boranes form complex spatial structures rather than chains of atoms. The lower boranes are gases (B2H6 and B4H10—tetraborane) and liquids (B5H9—pentaborane); the higher boranes (BioHn—decabo-rane, and others) are solids. Boranes are colorless; they have an unpleasant odor and are extremely poisonous. The lower boranes oxidize easily, are spontaneously inflammable in air at low temperatures (below 100° C), and are easily hydrolized by water to form boric acid, with the liberation of hydrogen. The higher boranes are more stable and less reactive. All boranes decompose to boron and hydrogen at temperatures greater than 700° C.
Diborane is produced by the interaction of boron chloride or boron fluoride with metal hydrides (LiH, NaH, NaBH4, LiA1H4) by the hydrogenation of boron chloride with hydrogen and by other means. The higher boranes are produced through the thermal decomposition of diborane.
The heat of combustion of the boranes is significantly higher than the heat of combustion for other compounds; for this reason, the boranes are of great interest as promising rocket fuels. The compound B2H6. is used to obtain elemental boron and for putting a boride covering on metals. Certain borane derivatives can be used to obtain heat-resistant polymers. Other boranes and certain of their derivatives are extremely toxic.
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V. L. VASILEVSKII