Species in which the central atom either does not have eight electrons in the valence shell or those which have 8 electrons in the valence shell but can expand their covalency beyond 4 due to the presence of \(d\)-orbital, are called electron deficient molecules. For example,
(i) In \(\mathrm{BCl}_3\), the central boron atom has only six electrons. Therefore, it is an electron deficient compound. As such, it accepts a pair of electrons from \(\mathrm{NH}_3\) to form an adduct


(ii) In \(\mathrm{SiCl}_4\), the central \(\mathrm{Si}\) atom has 8 electrons but it can expand its covalency beyond 4 due to the presence of vacant \(d\)-orbitals.