Question

Suggest reasons why the $BF$ bond lengths in $B{F}_3$ (130 pm) and ${B{F}_4}^{-}$ (143 pm) differ.

Answer 1

In $B{F}_3$ molecule, $B-F$ bond has partial double bond due to presence of $p\pi -p\pi$ backbonding between $B$ and $F$, which removes the electron deficienty of the molecule.
Due to this, $B-F$ bond length is reduced to 130 pm. When $B{F}_3$ changes to $B{F}_4^{-}$, the hybridization changes from $s{p}^2$ to $s{p}^3$.
The double bond character changes to single bond and the $B-F$ bond length becomes 143 pm.