Why is diborane classified as an electron-deficient compound?

Diborane ($B_2{H}_6$) is classified as an electron-deficient compound because it possesses fewer valence electrons than are required to form traditional two-center, two-electron covalent bonds between all its constituent atoms.1

To link its eight atoms together, the molecule would require at least seven bonds. However, diborane has only 12 valence electrons (3 from each of the two boron atoms and 1 from each of the six hydrogen atoms), which is sufficient to form only six standard covalent bonds.1

To compensate for this deficiency, diborane utilizes multicentre bonding, specifically three-centre, two-electron bonds. In this structure, a pair of electrons is delocalized over three atoms—two boron atoms and one bridging hydrogen atom ($B-H-B$)—rather than being restricted to a bond between just two atoms. This unique bonding arrangement allows the cluster of atoms to remain stable despite the lack of enough electrons to satisfy a conventional Lewis structure.231