Experimental and theoretical studies of the molecular and crystal structures of trialkoxy- and chlorodialkoxy-stibanes

The molecular structures of triisopropoxystibane, Sb((OPr)-Pr-i)(3), and chlorodiisopropoxystibane, SbCl((OPr)-Pr-i)(2), were determined in the solid state by single crystal X-ray diffraction. Sb((OPr)-Pr-i)(3) forms discrete centrosymmetric dimers in the solid state via Sb . . .O-Sb interactions, leading to pseudo trigonal bipyramidal configurations of the four co-ordinate Sb atoms, while SbCl((OPr)-Pr-i)(2) forms chains via Sb . . .O-Sb and Sb . . . Cl-Sb bridges, resulting in five-co-ordinate Sb atoms with pseudo octahedral configurations. Comparison of the solid state structures and the density functional optimized molecular structures of Sb(OMe)(3), SbCl(OMe)(2) and their dimers revealed a steady increase of the average Sb-O bond lengths with the co-ordination number of Sb, and mutual trans effects of the ligands. Standard enthalpies of dimer formation from density functional calculations are -23.8 and -69.7 kJ mol(-1) for [Sb-2(mu -OMe)(2)(OMe)(4)] and [Sb2Cl2(mu -OMe)(2)(OMe)(2)], respectively, and -42.7 kJ mol(-1) for [Sb-2(mu -Cl)(2)(OMe)(4)]. A natural bond orbital analysis reveals that n(O)-sigma*(Sb-O) and n(O)-sigma*(Sb-Cl) interactions are the main contributions to the inter-monomer bonding in the O-bridged dimers, of Sb(OMe)(3) and SbCl(OMe)(2), respectively, while n(Cl)-sigma*(Sb-O) plays no significant role in the Cl-bridged dimer of SbCl(OMe)(2). IR and Raman spectra of Sb((OPr)-Pr-i)(3) indicated molecular association in the solid and liquid phase, but dissociation into monomers in non-polar solvents.