Density functional theory and MP2 calculations have been performed on the Lewis acid-base complexes HnF3-nN --> BH3 and H3N --> BHnF3-n where n = 0-3. Predicted equilibrium structures, binding energies, and charge transfers have been correlated to the degree of substitution on the base and on the acid separately. Calculations on the symmetrically substituted complexes H2FN --> BH2F and F3N --> BF3 have also been included. It is found that successive fluorine substitutions on nitrogen reduce the complex binding energies. The same trend is found for successive substitutions on boron. The findings are interpreted in terms of rehybridizations of the nitrogen lone-pair orbital, changes in the HOMO-LUMO gap, and back-donation to the p pi orbital on boron.