Lewis acidity of group 14 tetrahalides in gas phase

Donor-acceptor complexes of silicon, germanium and tin tetrahalides with nitrogen-containing donors MX₄·nL (M=Si,Ge,Sn; X=F,Cl,Br; L=NH₃, Py, 2,2′bipy, 1,10-phen) have been theoretically studied at the B3LYP/DZP level of theory. It was found that some of the complexes possess negative Gibbs free energies, i.e. their decomposition into components is thermodynamically favorable. At the same time the formation of normal covalent (by means of the bond length) donor-acceptor bond was evidenced. Scanning of the potential energy surface (PES) revealed that such complexes are metastable species with barriers to dissociation 4-60 kJ mol^-1. Energies of the element-nitrogen donor-acceptor bond are strongly dependent on the coordination polyhedron type; however, for the given type of the coordination polyhedron they are independent on the nature of the donor molecule (NH₃, Py, bipy, phen). It is shown that the reorganization energy of the acceptor molecule (36-280 kJ mol^-1) governs the stability of the complexes. The reorganization energies of the donor molecules (1-32 kJ mol^-1) are much smaller. Dissociation enthalpies of the complexes decrease in order Sn>Ge>Si, in line with increasing reorganization energy of the acceptor. At the same time, the donor-acceptor bond energy changes nonmonotonically: Ge<Si<Sn. The values of the gas phase dissociation enthalpy should not be used to estimate the metal-nitrogen bond energy. On the basis of theoretical studies, the most promising candidates for stable gaseous complexes of group 14 halides are complexes of tin tetrahalides with bidentate donor ligands, such as SnF₄phen.