Donor-acceptor complexes MX 3 -D (M ) Al, Ga, In; X ) F, Cl, Br, I; D ) YH 3 , PX 3 , X -; Y ) N, P, As) and their components have been studied using self-consistent field and hybrid Hartree-Fock/density functional (B3LYP) methods with effective core potentials. The theoretical dissociation energies of the MX 3 -D complexes decrease in the orders F > Cl > Br > I, Al > Ga < In, and N . P g As for all investigated complexes. The calculated (B3LYP/LANL2DZP) dissociation energies for ammonia adducts are on average 7 kJ mol -1 higher than those from experiment. There is no correlation between the dissociation energy and the degree of charge transfer. Complexes of ammonia and metal fluorides have mostly ionic metal-donor bonds, while the other donor-acceptor adducts are mostly covalently bonded. In addition, a significant charge redistribution between the terminal atoms leads to further electrostatic stabilization of ammonia adducts. Coulomb interactions destabilize MX 3 -PX 3 complexes, and despite some experimental indications, the existence of these particular complexes in the gas phase is improbable. Distortion of MX 3 from planarity under complex formation leads to decreasing X-M-X angles. These decreasing angles correlate well with increasing M-X bond lengths. For all investigated MX 3 -Xsystems a strong correlation of the MX 3 -Xdissociation energy with the M-X bond length increase is found. Correlations between the pyramidal angle X-M-Y and the length of the adjacent M-Y bond have been found for each donor atom Y. All observed trends in structural and thermodynamic properties are qualitatively explained on the basis of a simple electrostatic model.

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Thermodynamic characteristics of vaporization and gas-phase dissociation of the complexes GaCl 3 pyz and GaCl 3 pyzGaCl 3 (pyz is pyrazine) were determined by static tensimetry with a membrane zero gage. Structural and thermodynamic characteristics of the complexes were calculated by the B3LYP/pVDZ quantum-chemical method.

Donor-acceptor complexes of silicon halides with ammonia, pyridine, and 2,2Јbipyridine SiX 4 ⅐ nD (X ϭ F, Cl, Br) have been studied at the B3LYP/pVDZ level of theory. Energies of the donor-acceptor bond have been estimated taking into account the reorganization energy of the donor and acceptor fragments and basis set superposition error correction. Despite of the very low (or even negative) dissociation energy of SiX 4 ⅐ nD into free fragments, the Si-N bonding in all complexes is rather strong (75-220 kJ mol Ϫ1 ). It is the reorganization energy of the acceptor SiX 4 (75-280 kJ mol Ϫ1 ) that governs the dissociation energy of the complex. Thus, in contrast to the complexes of group 13 halides, the reorganization effects are crucial for the complexes of group 14 halides, and their neglecting leads to erroneous conclusions about the strength of the donor-acceptor bond in these systems.