Donor-acceptor complexes of ammonia and formaldehyde with MX(3) (M = B, Al and X = H, F, Cl) have been studied using density functional methods. The optimized geometries and the formation energies of the complexes have been compared with ab initio results. The interaction between Lewis acids and bases has been analyzed in terms of steric and electronic effects. A different Lewis acidity scale is obtained depending on the Lewis base. This result has been rationalized in terms of the energy necessary to distort the Lewis acid molecules.