Solution:
Diborane (B2H6) is an electron-deficient molecule. Usually, 14 valence electrons required for the formation of eight bonds. But in diborane, 12 valence electrons form eight bonds in which each boron contributes three valence electrons and each hydrogen contributes one valence electron.
In AlH3, there are six electrons in which three electrons contributed by Al and each hydrogen contributes one valence electron. As AlH3 has incomplete octet, therefore it is known as electron deficient specie.
Si has four valence electrons and form four covalent bonds with four hydrogen atoms. Octet of Si in SiH4 is also complete. Therefore, SiH4 is not electron deficient.
In GaH3, Ga shares three valence electrons and each hydrogen shares one electron. There are a total of six electrons in GaH3 instead of 8. Therefore, GaH3 is electron deficient.
