Chemistry - Complete Pblock- Question with Solution | TestHub

Due to effective $\mathrm{p}\pi -\mathrm{p}\pi$ back bonding between B & F in ${\mathrm{BF}}_3$, it becomes the weakest Lewis acid. In the case of other halides, the less effective $\mathrm{p}\pi -\mathrm{d}\pi$ back bonding further decreases as the size of halogen atom increases. This makes the vacant orbital of B more and more available for receiving the lone pair of electrons.