Why is BCl3 a stronger Lewis acid with respect to strong bases than BF3?

Geometries and bond dissociation energies of the complexes Cl3B-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 F3B-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 BCl3.