What are electron-deficient compounds? Are `BCl_(3)` and `SiCl_(4)` electron-deficient species? Explain.

Explanation

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-orbitals, are called electron deficient molecules. For example.
(i) In `BCl_(3)`, the central boron atom has only six electrons. Therefore, it is an electrons deficient compound.
As such, it accepts a pair of electrons from `NH_(3)` to form an adduct

(ii) In `SiCl_(4)`, the central Si atom has 8 electrons but it can expandits covalency beyond 4 due to the presence of vacant d-orbitals. Therefore in principle, `SiCl_(4)` should also be an electron-deficient molecule but, in fact it does not accept two more `Cl^(-)` ions to form `[SiCl_(6)]^(2-)` becauseof the following two reasons.
(i) Six large sized `Cl` atoms cannot be accommodated around small `Si` atom.
(ii) Due to bigger size of `Cl`, interaction between lone pairs of chlorine atom and the empty d-orbitals silicon atom is weak.