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

Abstract

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.

Coordination chemistry of the main group elements with phosphine, arsine and stibine ligands

“…DFT calculations have explored the Lewis acid-Lewis base bonding in Group 14 elements, with the emphasis on silicon complexes with nitrogen donors, and comparisons with germanium analogues [253][254][255][256]. More recently N-heterocyclic carbene adducts have been similarly modelled [257,258].…”

Toward a comprehensive definition of oxidation state (IUPAC Technical Report)

“…A criterion suggested by Haaland [30] can also be applied, which interrogates where the bonding electrons go when the bond is split thermally: If the split is heterolytic, the ionic approximation follows the direction of electrons; if the split is homolytic, the typical energy of the atom's valence orbital applies. Haaland's criterion can also be evaluated by quantum-chemical calculations [31], which are, however, more involved than those of orbital energy and mixing coefficient. A simple estimate is therefore desirable.…”

Effect of Substituents on the Strength of A−Cl-(A = Si, Ge, and Sn) Bonds in Hypervalent Systems: ACl5-, ACl4F-, and A(CH3)3Cl2-

“…Fleischer studied a range of substituted silane-Lewis base complexes with density functional methods similar to those employed here. 17 Halogenated four-coordinate silicon compounds have been studied using very high-level techniques by Chien, Li, and Ma. 18 …”