Question

The correct statement is

• A. $B{F}_3$ is the strongest Lewis acid among the other boron halides.
• B. $B{I}_3$ is the weakest Lewis acid among the boron halides
• C. There is maximum $p\pi$ - $p\pi$ back bonding in $B{F}_3$
• D. There is minimum $p\pi$ - $p\pi$ back bonding in $B{F}_3$

Answer 1

Answer: (C)

$B{F}_3$ is a weaker Lewis acid than other boron halides due to strong p$\pi$ - p$\pi$ back bonding in $B{F}_3$

In $B{F}_3$ each fluorine has completely unutilised 2p while boron has a vacant 2p orbital. Now since, both of these orbitals belong to same energy level (2p), they can overlap effectively as a result of which fluorine electrons are transferred into the vacant 2p orbital of boron resulting in the formation of an additional p$\pi$-p$\pi$ bond. As a result of back donation of electrons, the electron deficiency of boron reduced and hence, Lewis acidity is decreased.