Diborane Structure

Diborane, B_2H_6, is an electron-deficient molecule that cannot be described by ordinary two-centre two-electron bonds because it does not have enough valence electrons. Each boron is bonded to two terminal hydrogen atoms by normal bonds, but the two bridging hydrogen atoms are each shared between both boron atoms through three-centre two-electron bonds, often called banana bonds. In each bridge, a single pair of electrons binds three atoms, namely two borons and one hydrogen, accounting for the electron deficiency. The option of normal two-centre two-electron bonds cannot explain the bridges with insufficient electrons. The option of ionic bonds is wrong, since diborane is covalent and molecular. The option of metallic bonds does not apply to a discrete molecule. This bridged structure is a classic NCERT example of electron-deficient bonding in Group 13. Working through the logic step by step, rather than memorising the result, makes it clear why banana bond governs the behaviour seen here. Carefully relating the data to the governing principle ensures the reasoning remains valid even when the numbers or species in the question are changed. Plausibility check: diborane has twelve valence electrons but would need more for eight conventional bonds, so the bridging three-centre bonds are required, confirming the banana-bond description.