JEE Main · 2021 · 27 Aug 2021 · Shift 1 · Chemical Bonding and Molecular Structure

Back donation into boron is a $p\pi \text{-}p\pi$ interaction between a filled halogen $p$ orbital and the empty $2p$ orbital of boron, and it is strongest when the two orbitals are similar in size. Fluorine's $2p$ orbital matches boron's $2p$ orbital most closely, giving the best overlap and the strongest donation, whereas the larger, more diffuse $p$ orbitals of the heavier halogens overlap poorly. The strongest back donation occurs in $\mathrm{B}{\mathrm{F}}_3$.