Ab initio study of isomerism, structure, and stability of complex salt molecules of oxo- and thiodihalo- and thiodihydroborates and -aluminates Li[X2MY] (M = B, Al; X=F, Cl, H; Y=O,S)

Ab initio calculations of the potential energy surfaces in the vicinity of key structures of X2MY- planar trigonal anions (F2BO-, Cl2BO-, F2BS-, Cl2BS -, F2AlO-, Cl2AlO-, F2AlS-, Cl2AlS-, H2BO-, H2BS-, H2AlO-, and H2AlS-) and their lithium salts Li[X2MY] are performed in terms of the correlated MP4/6 -31(+)G**//MP2/6-31G* + ZPE(MP2/(6-31G*) approach. In these structures, the Li+ cation is coordinated to both halogen or hydrogen atoms X in the biden tate mode (structure 1, C-2v(b)), to the chalcogen atom Y in the monodentat e mode (structure 2, C-2v(m)), and to the chalcogen atom and one of the X a toms in the bidentate mode (structure 3, C-s(b')). Equilibrium geometric pa rameters and the frequencies and relative IR intensities of normal vibratio nal modes of isomers are calculated, and their relative energies and dissoc iation energies are determined. The bidentate structure 3 is the most stabl e for all the salts, except aluminates, in which structure 3 transforms wit hout barrier into structure 2 with the deformationally nonrigid Al-O-Li bri dge. In all the molecules, the second bidentate structure 1 corresponds to a local minimum lying 12-22 kcal/mol above the global minimum. For various coordination modes of X2MY- anions, their polarization and distortion in th e field of the Li+ cation are analyzed; trends in calculated parameters upo n the substitution of Al for B, Cl and H for F, S for O, and in other relat ed series are examined.