• # question_answer 22) Suggest reasons why the BF bond lengths in $B{{F}_{3}}$(130 pm) and $BF_{4}^{-}$ (143 pm) do differ?

$B{{F}_{3}}$has a planar structure as boron is in $sp$ -hybridized state. It has one empty p-orbital. F atom has lone pairs. On account of small size, a lone pair of F atom is transferred to B atom and thus forms a$p\pi -p\pi$ back bonding, i.e., double bond character comes and due to resonance in the molecule, the bond length is shorter. In $BF_{4}^{-}$ ion, B atom is $s{{p}^{3}}$ -hybridized and it has no p-orbital, i.e., back bonding or $p\pi -p\pi$ bonding is not possible. Thus, the bond length, B-F is larger as the bond is purely a single bond.