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Molecular “Floppyness” and the Lewis Acidity of Silanes: A Density Functional Theory Study
Author(s) -
Fleischer Holger
Publication year - 2001
Publication title -
european journal of inorganic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 1434-1948
DOI - 10.1002/1099-0682(200102)2001:2<393::aid-ejic393>3.0.co;2-z
Subject(s) - chemistry , lewis acids and bases , silanes , exothermic reaction , standard enthalpy of formation , density functional theory , thermochemistry , adduct , enthalpy , crystallography , ionic bonding , computational chemistry , silane , organic chemistry , thermodynamics , catalysis , ion , physics
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 = NH 3 , OH 2 , F − ) are shorter in hexa‐ than in pentacoordinated complexes. The molecular structures of the pentacoordinated Si complexes allowed for a mapping of an S N 2 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 SiH 2 X 2 (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 SiF 4 (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 SiCl 4 (py) 2 compared to SiH 4 (py) 2 and of GeF 4 (NH 3 ), [GeF 5 ] − and [GeF 6 ] 2− compared to the analogues silane adducts does not result from a stronger Lewis acid base interaction but from an increased “floppyness” of SiCl 4 and GeF 4 compared to SiH 4 and SiF 4 , respectively.