`BF_3` is a weaker Lewis acid than `BCl_3`, even though F is more electronegative than Cl. Explain.

Explanation

The B -atom in `BF_3 or BCl_3` has only six electrons in its valence shell and hence it is capable of accepting a pair of electrons to complete its octet. Therefore, both `BF_3 and BCl_3` act as Lewis acids. Because of equal sizes of the empty 2p - orbital of B and filled 2p -orbital of F, the lone pair of electrons of F is donated to the empty 2p-orbital (prr-prr back bonding) to a considerable extent and as a result, the electron deficiency of B decreases in `BF_3." In "BCl_3`, on the other hand, the size of the 3p -orbital of Cl containing the lone pair of electrons is much bigger than the empty 2p -orbital of B and hence donation of lone pair of electrons from Cl to B does not take place significantly. Therefore, the B atom in `BCl_3` s a stronger Lewis acid than `BF_3`.