Boron hydrides are quite interesting form the point of view of their structures. The determination of structure of boron hydries was such an intricate problem that William Lipscomb was awarded a Noble Prize in 1976 for elucidation of their structures.

The structure of diborane-the simplest boron hydride is quite interesting as there are not enough valence electrons to form the expected number of convent bonds. There are twelve valence electrons in B₂H₆ (three per boron atom and one fore each H atom) and for the molecule to have an ethane type structure a minimum of fourteen electrons are required as one electron pair is required to form each covalent bond.

Therefore, we may conclude that diborane cannot have ethane type structure. Hence, hydrogen bridged structure for diborane was proposed based on the following evidence.
(i)    Based on electron diffraction studies, it was concluded that diborane has hydrogen bridged structure as shown in.

The above structure depicts that there are two types of hydrogen atoms in diborane, namely, Ht type which form normal covalent bonds with boron and Hb type which form bridge between the two boron atoms. The two-bridge hydrogen atoms are in a plane at right angle to the rest of the molecule. Therefore, there is no free rotation between the two boron atoms.

(ii)    Specific heat measurements also confirmed that there is no free rotation between the two boron atoms.

(iii)    Raman and proton nuclear magnetic resonance (NMR) spectra confirmed that four hydrogen atoms in dibroane are different than the other two hydrogen atoms.

(iv)    Methylation of diborane also confirmed that four hydrogen atoms in diborane are different than the other two hydrogen atoms as not more than four hydrogen atoms can be methylated to give (CH₃)₄B₂H₂.

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