The SO 3 -NH 3 - n X n (n = 0-3; X = F, Cl) acceptor-donor complexes have been investigated at the G2MP2 level of theory. The G2MP2 results show that the successive fluorine and chlorine substitution on the nitrogen decreases the stability of SO 3 -NH 3 - n F n and SO 3 -NH 3 - n Cl n complexes, in accord with the decreasing basicity of the NH 3 - n F n and NH 3 - n Cl n ligands. The natural bond orbital (NBO) partitioning schemes show that the shortening of the N-F and N-Cl bond lengths, upon complexation, is due to an increasing s character in these bonds.