Electron deficient compounds are those in which the total number of atomic orbitals on all the atoms of the compound is more than the total number of valence shell electrons.
Boranes are electron deficient compounds:
B = 1s22s22px12py2pz
H = 1s1
* number of orbitals from 2 B = 2 X 4 = 8
number of orbitals from 6 H = 6 X 1 = 6
Therefore, total number of orbitals in diborane is 14, which can accommodate (14X2) = 28 electrons.
But, the total valence shell electrons in diborane due to 2B and 6 H atoms is = 12. According to definition of deficient compounds,
B2H6 is an electron deficient compound.
Similarly, you can prove the following boranes are electron deficient compounds( Try Yourself)
B5H9, B4H10 and B5H11