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Concept
Diborane
Summary
Diborane(6), commonly known as diborane, is the chemical compound with the formula B2H6. It is a toxic, colorless, and pyrophoric gas with a repulsively sweet odor. Given its simple formula, borane is a fundamental boron compound. It has attracted wide attention for its electronic structure. Several of its derivatives are useful reagents.
The structure of diborane has D2h symmetry. Four hydrides are terminal, while two bridge between the boron centers. The lengths of the B–Hbridge bonds and the B–Hterminal bonds are 1.33 and 1.19 Å respectively. This difference in bond lengths reflects the difference in their strengths, the B–Hbridge bonds being relatively weaker. The weakness of the B–Hbridge compared to B–Hterminal bonds is indicated by their vibrational signatures in the infrared spectrum, being ≈2100 and 2500 cm−1 respectively.
The model determined by molecular orbital theory describes the bonds between boron and the terminal hydrogen atoms as conventional 2-center 2-electron covalent bonds. The bonding between the boron atoms and the bridging hydrogen atoms is, however, different from that in molecules such as hydrocarbons. Each boron uses two electrons in bonding to the terminal hydrogen atoms and has one valence electron remaining for additional bonding. The bridging hydrogen atoms provide one electron each. The B2H2 ring is held together by four electrons forming two 3-center 2-electron bonds. This type of bond is sometimes called a "banana bond".
B2H6 is isoelectronic with C2H62+, which would arise from the diprotonation of the planar molecule ethylene. Diborane is one of many compounds with such unusual bonding.
Of the other elements in group IIIA, gallium is known to form a similar compound digallane, Ga2H6. Aluminium forms a polymeric hydride, (AlH3)n; although unstable, Al2H6 has been isolated in solid hydrogen and is isostructural with diborane.
Extensive studies of diborane have led to the development of multiple synthesis routes. Most preparations entail reactions of hydride donors with boron halides or alkoxides.
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