Question

Why is `"AlF"_3` not-as-often used a good Lewis acid catalyst?

Answer 1

Because (paradoxically), the Lewis acidic centre in `AlF_3`, that is the aluminum atom, is not as Lewis-acidic as the metal centre in `AlCl_3` or `AlBr_3`

Explanation:

The experimental fact is that both aluminum chloride and aluminum bromide are stronger Lewis acids that aluminum fluoride; in fact `AlBr_3` is one of the strongest Lewis acids.

How to rationalize this observation?

While fluorine is a more electronegative element than both chlorine and bromine, the fluoride ion, bound to aluminum, has lone pairs of the right size and the right shape to shunt electron density to the aluminum centre, thus moderating its Lewis acidity. The corresponding lone pairs of `Br` or `Cl` are larger and more diffuse, and when bound to aluminum do not donate electron density as effectively to the metal centre.

A similar rationale applies to the reduced Lewis acidity of `BF_3` with respect to `BCl_3` and `BBr_3`.