Why Is BCl₃ a Stronger Lewis Acid with Respect to Strong Bases than BF₃?

Geometries and bond dissociation energies of the complexes Cl ₃B-NH₃ and F₃B-NH₃ have been calculated using DFT (PW91) and ab initio methods at the MP2 and CCSD(T) levels using large basis sets. The calculations give a larger bond dissociation energy for Cl₃B-NH₃ than for F3B-NH3. Calculations of the deformation energy of the bonded fragments reveal that the distortion of BCl₃ and BF₃ from the equilibrium geometry to the pyramidal form in the complexes requires nearly the same energy. The higher Lewis acid strength of BCl₃ in X₃B-NH₃ compared with BF₃ is an intrinsic property of the molecule. The energy partitioning analysis of Cl₃B-NH₃ and F ₃B-NH₃ shows that the stronger bond in the former complex comes from enhanced covalent interactions between the Lewis acid and the Lewis base which can be explained with the energetically lower lying LUMO of BCl ₃.