Question

Among the halides
$i.BC{l}_3$
$ii.AlC{l}_3$
$iii.GaC{l}_3$
$iv.InC{l}_3$
The order of decreasing Lewis acid character is:

• A. i, ii, iii, iv
• B. ii, iii, iv, i
• C. iii, iv, ii, i
• D. iv, iii, ii, i

Answer 1

The correct option is D iv, iii, ii, i

The acidic charecter of chlorides increases down the group. $BC{l}_3$ and $InC{l}_3$ are the weakest and the strongest Lewis acids respectively.

This is because in $BC{l}_3$ the pπ-pπ bonding is the strongest, making it more stable and hence a weak Lewis acid.

Backbonding:
In $BC{l}_3$, boron has vacant $2p$-orbital and each chlorine has full filled unutilised $3p$-orbitals.
Chlorine transfers two electrons to vacant $2p$-orbitals of boron, thus forming $p\pi -p\pi$ bond.

This type of bond has a double bond character and is known as dative or backbonding.

It reduces the electron deficiency of the boron atom and so its Lewis acid character decreases.