JEE Main · 2021 · 17 Mar 2021 · Shift 1 · Equilibrium

A Lewis base must carry a lone pair that it can donate. Chlorine in $\mathrm{Cl}{\mathrm{F}}_3$ retains two lone pairs, nitrogen in $\mathrm{N}{\mathrm{F}}_3$ one and sulphur in $\mathrm{S}{\mathrm{F}}_4$ one, so each of these can give a pair away. In $\mathrm{PC}{\mathrm{l}}_5$ all five valence electrons of phosphorus are used in the five P–Cl bonds, leaving the central atom with no lone pair at all; its vacant $d$ orbitals instead let it accept a pair, which is why it adds chloride to form $\mathrm{PC}{\mathrm{l}}_6^{-}$. The compound that cannot act as a Lewis base is $\mathrm{PC}{\mathrm{l}}_5$.