Quantum mechanical calculations at the MP2/TZ2P level of theory predict geometries and bond energies of donor-acceptor complexes of the Lewis acids BH3, BF3, BCl3, AlCl3, and SO2 which are in very good agreement with experimental gas-phase values. Strong donor-acceptor bonds are calculated for the boron complexes OC-BH3 (D-0(298) = 25.1 kcal/mol), H3N-BH3 (D-0(298)= 30.7 kcal/mol), Me(3)N-BH3 (D-0(298) = 41.1 kcal/mol), H3N-BF3 (D-0(298) = 22.0 kcal/mol), Me(3)N-BF3 (D-0(298) = 32.9 kcal/mol), H3N-BCl3 (D-0(298) = 29.7 kcal/mol), and Me(3)N-BCl3 (D-0(298) = 40.5 kcal/mol). Weakly bound van der Waals complexes are predicted for OC-BF3 (D-0(298) = 4.7 kcal/mol), HCN-BF3 (D-0(298) = 7.2 kcal/mol), MeCN-BF3 (D-0(298) = 9.1 kcal/mol), OC-BCl3 (D-0(298) = 4.0 kcal/mol), and MeCN-BCl3 (D-0(298) = 6.4 kcal/mol). Intermediate dissociation energies are calculated for the BF, complexes with Me(2)O (D-0(298) = 17.3 kcal/mol), benzaldehyde (D-0(298) = 13.0 kcal/mol), and 2-methylacrolein (D-0(298) = 12.8 kcal/mol). The strongest donor-acceptor bond is calculated for Me(3)N-AlCl3, (D-0(298) = 49.3 kcal/ mol). A strong bond is also predicted for EtCClO-AlCl3 (D-0(298) = 24.8 kcal/mol), while the complex Me(3)N-SO2 is more weakly bound (D-0(298) = 15.5 kcal/mol). The bond lengths of the Lewis acids are longer in the complexes than in the isolated molecules. A good correlation is found between the calculated bond strengths of the BF3 complexes and the lengthening of the B-F bond. The NBO partitioning scheme suggests that there is no correlation between the charge transfer and the bond strength. The topological analysis of the electron density distribution shows that the donor-acceptor bonds of the strongly bound boron complexes have significant covalent contributions, while the weakly bound boron complexes are characterized by electrostatic interactions between the Lewis acid and base. However, the nature of the strongly bound AlCl3 complexes is different from that of the strongly bound boron complexes. The strongest donor-acceptor bond calculated for Me(3)N-AlCl3 is characterized by electrostatic interactions and very little covalent contributions. The bond shortening of the donor acceptor bonds between the gas phase and the solid state is calculated to be mainly due to short-range dipole-dipole interactions. The geometry-optimized dimer and tetramer of H3N-BH3 and the dimer of H3N-BF3 have significantly shorter B-N bonds than the monomer.