B-F bond length in $\mathrm{BF}_{3}$ is 130 pm . B-F bond length in $\mathrm{BF}_{4}^{-}$is expected

Question

B-F bond length in $\mathrm{BF}_{3}$ is 130 pm . B-F bond length in $\mathrm{BF}_{4}^{-}$is expected to be (in pm)\

Professor PC Thomas Classes · Answer

In $\mathrm{BF}_{3}$ the B-F bonds possess double character due to backbonding. In $\mathrm{BF}_{4}^{-}$double bond character is lost due change in hybridization of boron from $\mathrm{sp}^{2}$ to $\mathrm{sp}^{3}$. Therefore B-F bond length is greater in $\mathrm{BF}_{4}^{-}$than that in $\mathrm{BF}_{3}$.\