Question

discuss the nature of bonding in diborane

Answer 1

Diborane adopts a D2h structure containing four terminal and two bridging hydrogen atoms. The model determined by molecular orbital theory indicates that the bonds between boron and the terminal hydrogen atoms are conventional 2-center, 2-electron couldn't bonds. The bonding between the boron atoms and the bridging hydrogen atoms is, however, different from that in molecules such as hydrocarbons. Having used two electrons in bonding to the terminal hydrogen atoms, each boron has one valance electron remaining for additional bonding. The bridging hydrogen atoms provide one electron each. Thus the B2H2 ring is held together by four electrons, an example of 3-center 2-center electric bonding. This type of bond is sometimes called a 'banana bond'. The lengths of the B-Hbridge bonds and the B-Hterminal bonds are 1.33 and 1.19 Å respectively, and this difference in the lengths of these bonds reflects the difference in their strengths, the B-Hbridge bonds being relatively weaker. The weakness of the B-Hbridge vs B-Hterminal bonds is indicated by their vibrational signatures in the infrared spectrum, being ~2100 and 2500 cm−1, respectively.The structure is isoelectronic with C2H62+, which would arise from the diprotonation of the planar molecule ethane.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, dig all a new, Ga2H6. Aluminium forms a polymeric hydride, although unstable Al2H6 has been isolated in solid hydrogen and is isostructural with diborane.