What are electron deficient compounds? Are `BCl_3` and `SiCl_4` electron deficient species? Explain.

Explanation

Electron deficient compounds are those in which the central atom either does not have 8 valence electrons or in which the central atom has 8 valence electrons but can expand its valency beyond 4 due to presence of vacant d -orbitals.
`BCl_3` is an electron deficient molecule as central B atom has six electrons in its valence shell. So, it can accept a pair of electrons from Lewis bases like `NH_3` to form an adduct.

The central Si atom in `SiCl_4` has 8 electrons in its valence shell but it can e>..-pand its covalency beyond 4 due to presence of vacant d -orbitals. Therefore, in principle, `SiCl_4` should also act as an electron-deficient compound. However, it cannot form `[SiCl_6]^(-2)` by accepting two `Cl^(-)` ions and this is because 1. small Si -atom cannot accommodate six large sized Cl-atoms around itself and 2. the interaction between lone pairs of Cl -atom and empty p -orbitals of Si atom is weak.