Electron deficient hydrides do not have sufficient electrons to form normal covalent bonds. Therefore, to make up this deficiency they generally exist in polymeric forms such as \(\mathrm{B}_2 \mathrm{H}_6, \mathrm{~B}_4 \mathrm{H}_{10},\left(\mathrm{AlH}_3\right)\) n, etc. Further to make up this deficiency of electrons, they react with many metals, non-metals and their compounds. Hence, electrondeficient hydrides are very reactive as shown below:
\(\mathrm{B}_2 \mathrm{H}_6(\mathrm{~g})+3 \mathrm{O}_2(\mathrm{~g}) \longrightarrow \mathrm{B}_2 \mathrm{O}_3(\mathrm{~s})+3 \mathrm{H}_2 \mathrm{O}(\mathrm{g})\) \(2 \mathrm{NaH}(\mathrm{s})+\mathrm{B}_2 \mathrm{H}_6(\mathrm{~g}) \stackrel{\text { Diethylether }}{\longrightarrow} \underset{\text { Sod. borohydride }}{2 \mathrm{Na}^{+}\left[\mathrm{BH}_4\right]^{-}(\mathrm{s})}\)
Being electron deficient compounds, they act as Lewis acids and thus form complex with Lewis bases. For example.