Preservation of the Total Order of Axial X–SI and SI←N Bonds (X = F, Cl, O, N, C) in Silatrane Molecules

Abstract

Seventy experimental (XRD and GED) structures of silatranes XSi(OCH₂CH₂)₃N are used to map the reaction pathway of S_N2-type substitution at tetrahedral silicon following the Bürgi – Dunitz approach. It is shown that only the use of high-quality XRD structures (R-factor <5%) allows observing the hyperbolic-type relationship between the axial X–Si and Si←N bond lengths in silatrane molecules. A unified line for the conservation of the total order of these bonds is obtained using the Levenberg – Marquardt method, and it is shown that the sum of the orders of these bonds is equal to unity for various X. This provides important evidence for the three-center four-electron character of the X–Si←N fragment and the presence of a covalent contribution to the Si⋯N interaction in silatrane molecules.