A COMPUTATIONAL ANALYSIS OF THE BONDING IN BORON-TRIFLUORIDE AND BORON-TRICHLORIDE AND THEIR COMPLEXES WITH AMMONIA

Complexation energies for the interactions of BF3 and BCl3 with NH3 ha ve been calculated at the ab initio Hartree-Fock and MP2 levels of the ory, using large polarized basis sets. The formation of H3N.BCl3 is fo und to be favored by 4.27 kcal/mol over H3N.BF3 at the MP2 level. This is in agreement with the experimental observation that the Lewis acid ities of the boron trihalides increase in the order BF3 < BCl3 < BBr3. Calculated atomic charges and molecular electrostatic potentials show the boron to be much more positive in BF3 than in BCl3, as would be e xpected from the respective electronegativities of fluorine and chlori ne. These results and the relevant ppi-ppi overlap integrals do not su pport using the concept of back-bonding and consequent stabilization t o explain the trend in Lewis acidities. As an alternative explanation, it is suggested that this trend reflects the importance of Lewis base --> BX3 charge transfer in these complexes and the fact that the abil ity to accept the charge, as indicated by charge capacities, increases in the order BF3 < BCl3 < BBr3.