Question

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

Answer 1

The B–F bond length in $B{F}_3$ is shorter than the B–F bond length in $B{F}_4^{-}$ . $B{F}_3$ is an electron- deficient species. With a vacant p-orbital on boron, the fluorine and boron atoms undergo pπ–pπ back-bonding to remove this deficiency. This imparts a double bond character to the B–F bond.

This double-bond character causes the bond length to shorten in $B{F}_3$ (130 pm). However, when $B{F}_3$ coordinates with the fluoride ion, a change in hybridisation from $s{p}^2$ (in $B{F}_3$) to $s{p}^3$ (in $B{F}_4^{-})$ occurs. Boron now forms 4σ bonds and the double-bond character is lost. This accounts for a B–F bond length of 143 pm in $B{F}_4^{-}$ ion.