In diborane, 8 covalent bonds and 12 electrons are present. But it requires a total 14 electrons to form bonding. It lacks 2 electron pairs and forms a 3c−2e−bond. Thus B2H6 is electron deficient. 
Related Theory The boron atom is known to be sp3 hybridized and has four hybrid orbitals. The two electrons of the hybrid orbitals in each of the boron atoms form 2 bonds with the Is hydrogen atoms. The two atoms of boron left with that of each unpaired electron orbital and empty orbital forms the two bridging (B−H−B) bonds with that of the two Is hydrogen atoms, is also called as the banana bond.