Question

The Lewis acidity of $B{F}_3$ is less than $BC{l}_3$ even though fluorine is more electronegative than chlorine. It is due to

• A. stronger $2p(B)-2p(F)\sigma$ - bonding
• B. stronger $2p(B)-2p(F)\pi$ - bonding
• C. stronger $1p(B)-3p(Cl)\sigma$ -bonding
• D. stronger $2p(B)-3p(Cl)\pi$ - bonding

Answer 1

Answer: (B)

Boron and fluorine do not have $d$ -orbitals. Hence, both of these participate in strong $2p(B)-2p(F)$ back $\pi$ -bonding. On the other hand, due to large size and availability of vacant $d$ -orbitals, Cl does not participate in such type of back $\pi$ -bonding. Hence, $B{F}_3$ is less acidic than $BC{l}_3$.