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

Geometries and bond dissociation energies of the complexes Cl(3)B[bond]NH(3) and F(3)B[bond]NH(3) 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 Cl(3)B[bond]NH(3) than for F(3)B[bond]NH(3). Calculations of the deformation energy of the bonded fragments reveal that the distortion of BCl(3) and BF(3) from the equilibrium geometry to the pyramidal form in the complexes requires nearly the same energy. The higher Lewis acid strength of BCl(3) in X(3)B[bond]H(3) compared with BF(3) is an intrinsic property of the molecule. The energy partitioning analysis of Cl(3)B[bond]NH(3) and F(3)B[bond]NH(3) 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).