THE PI-DONOR ABILITY OF THE HALOGENS IN CATIONS AND NEUTRAL MOLECULES- A THEORETICAL-STUDY OF AX(3)(+), AH(2)X(+), YX3, AND YH2X (A=C, SI,GE, SN, PB, Y=B, AL, GA, IN, TL, X=F, CL, BR, I)

Quantum mechanical ab initio calculations at the MP2/VDZ+P level of th eory (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 t he 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 t he 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 st udy of Olah and co-workers (J. Am. Chem. Sec. 1996, 118, 3580). The sa me 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 compou nds by the halogens is less uniform, because the sigma- and pi-donatio n show different trends. The energies of isodesmic reactions and the X (3)A(+)-OH2 complexation energies predict for the thermodynamic stabil ization of the Y-conjugated cations the same trend as the pi-donor abi lity, i.e. F < Cl < Br < I. The energy differences are much less for t he 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 positiv e charge at the carbon atom of CF3+. The complexation energies of the boron halides BX3 show Little differences for X3B-OH2 among the haloge ns, although the p(pi) population at boron increases clearly from fluo rine to iodine. The hydride affinity, however, increases strongly fi o m 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 thermodyn amic stabilization or destabilization is clearly less than for C+ and B, respectively.