Suggest a reason why B-F bond length in `BF_3` (130 pm) and `BF_4^(-)` (143 pm) differ?

Answer

B-F bonds in `BF_3` are formed by `sp^3` hybridised orbitals of boron. The 2p-orbital of fluorine containing lone pair overlaps laterally to a small extent with the empty 2p orbital of boron.Thus B-F bonds assumes slight double bond charcter. B-F bonds in `BF_4^(-)` are formed by `sp^3` hybridised orbitals of boron and there is no empty p-orbital. There is no possibility of back bounding in `BF_4^(-)`. Hence B-F bonds in `BF_4^(-) are ordinary single bonds.