Why is BCl3 a strongerLewis acid than BF3 ?

The strength of a Lewis acid is a measure of its ability to attract a pair of electrons on a molecule that is behaving as a Lewis base. Fluorine is more electronegative than chlorine, so it appears that three fluorine atoms should withdraw electron density from the boron atom, leaving it more positive.This would also happen to some extent when the peripheral atoms are chlorine, but chlorine is less electronegative than fluorine. On this basis, we would expect BF₃ to be a stronger Lewis acid. However,in the BF₃ molecule, the boron atom uses sp2 hybrid orbitals, which leaves one empty 2p orbital that is perpendicular to the plane of the molecule. The fluorine atoms have filled 2p orbitals that can overlap with the empty 2p orbital on the boron atom to give some double bond character (Back Bond) to the B–F bonds.

 

As a result of the contribution by resonance structures having some double bond character, the boron atom in BF₃ is not as electron deficient as it is in BCl₃. And hence BCl₃ a stronger Lewis acid than BF₃