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RESEARCH PRODUCT

Molecular “Floppyness” and the Lewis Acidity of Silanes: A Density Functional Theory Study

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A comprehensive set of Lewis acid-base adducts of silanes was investigated by means of the density functional theory geometry optimization [B3LYP/6−31G(d)], and thermochemical calculations, [B3LYP/6−311+G(2d,p)//B3LYP/6−31G(d)]. Complex formation was found to weaken Si−Cl and Si−Br bonds more than Si−F or Si−H bonds. Comparable distances between Si and a Lewis base L (L = NH3, OH2, F−) are shorter in hexa- than in pentacoordinated complexes. The molecular structures of the pentacoordinated Si complexes allowed for a mapping of an SN2 reaction pathway by correlating the lengths of the Si−X and Si−L bonds. Complex formation was found to be exothermic for most of the coordination compounds, and the analysis of the natural atomic charges revealed a high ionic character of the dative bonds. Formation of the anionic complexes and of SiH2X2(py)2 is most exothermic with X = Cl or Br and otherwise most exothermic with X = F. Only small enthalpy differences were found between the trans and cis configurations of SiF4(py)2. The standard free enthalpy of complex formation is negative only for complexes between halosilanes and F−, i.e. all other silane Lewis base adducts are thermodynamically unstable under standard conditions with respect to dissociation. It is inferred that the existence of some of the silicon complexes in the solid state or in solution is caused by stabilizing intermolecular forces, and silanes are classified as very weak Lewis acids. The thermochemistry of complex formation was analyzed in terms of molecular “floppyness” of the silanes and the energy of interaction between the deformed silane and the Lewis base. The enhanced complex stability of SiCl4(py)2 compared to SiH4(py)2 and of GeF4(NH3), [GeF5]− and [GeF6]2− compared to the analogues silane adducts does not result from a stronger Lewis acid base interaction but from an increased “floppyness” of SiCl4 and GeF4 compared to SiH4 and SiF4, respectively.