Quantum mechanical ab initio calculations at the MP2/VDZ+P level of theory (for some molecules at MP2/VTZ+D+P) are reported for the cations AX(3)(+) and AH(2)X(+) (A = C, Si, Ge, Sn, Pb; X = F, Cl, Br, I) and the isoelectronic neutral Lewis acids YX3 and YH2X (Y = B, Al, Ga, In, Tl; X = F, Cl, Br, I). The st-donor ability of the halogens given by the p(pi) population at atom A increases for all cations with F < Cl < Br < I, which is the opposite to what has been reported in a recent study of Olah and co-workers (J. Am. Chem. Sec. 1996, 118, 3580). The same trend is found at the central atom Y of most neutral compounds, but the increase is less pronounced than for the cations. The calculated order of the energetic stabilization of the cations and neutral compounds by the halogens is less uniform, because the sigma- and pi-donation show different trends. The energies of isodesmic reactions and the X(3)A(+)-OH2 complexation energies predict for the thermodynamic stabilization of the Y-conjugated cations the same trend as the pi-donor ability, i.e. F < Cl < Br < I. The energy differences are much less for the isodesmic reactions of the singly substituted AH(2)X(+) than for AX(3)(+). Fluorine has a higher stabilizing effect in CH2F+ than in CF3+, because the sigma-withdrawing strength of F leads to a large positive charge at the carbon atom of CF3+. The complexation energies of the boron halides BX3 show Little differences for X3B-OH2 among the halogens, although the p(pi) population at boron increases clearly from fluorine to iodine. The hydride affinity, however, increases strongly fi om BF3 to BI3 and from BH2F to BH2I. The heavy-atom homologues of C+, i.e. Si+, Ge+, Sn+, Pb+, and those of B, i.e. Al, Ga, In, Tl, show the same trends for the halogens, but the p(pi) donation and the thermodynamic stabilization or destabilization is clearly less than for C+ and B, respectively.