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  4. The Lewis acidity of BF3 is less than BCl3 even though fluorine is more electronegative than chlorine. It is due to

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Q. The Lewis acidity of $BF_3$ is less than $BCl_3$ even though fluorine is more electronegative than chlorine. It is due to

UPSEEUPSEE 2016Chemical Bonding and Molecular Structure

Solution:

Boron and fluorine do not have $d$ -orbitals. Hence, both of these participate in strong $2 p(B)-2 p(F)$ 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, $BF _{3}$ is less acidic than $BCl _{3}$.