Among boron halides, which is the strongest Lewis is acid?

Step-by-step solution

Generally, as the electronegativity increases, acidic character increases, but in case of boron halides. BI ₃ is the most acidic, i.e. strongest Lewis acid. This is because there is back donation of electrons from F -atom to boron in case of BF ₃ . This makes BF ₃ less electron deficient. The tendency of back donation decreases as the size of halogen atom increases due to large energy difference. Hence, BI ₃ is the most electron deficient and thus, the strongest Lewis acid.