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

Suggest a reason why B-F bond length in BF_3 (130 pm) and BF_4^(-) (14

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