Conservation of the Total Order of the Axial SiSi and N → Si Bonds in Oligosilanylsilatranes

Structural data from X-ray diffractometry (XRD) of oligosilanylsilatranes with a common coordination center SiSiO₃N are used to map the S_N2 substitution pathway at silatrane Si^IV atom using the Bürgi-Dunitz method of structural correlation (MSC). A classical hyperbola-like reaction coordinate is featured using the nonlinear least squares method. This representation perfectly corresponds to the ideal MSC pattern imposed by the conservation of the total order of the axial bonds in silatranes. At the analytical level, it describes the inverse relationship of the internuclear distances SiSi and N → Si of the axial fragment N → SiSi and provides an experimental (XRD) support for the presence of a covalent contribution in the dative contact N → Si in oligosilanylsilatranes. Theoretical bond orders determined by various methods are analyzed from the point of view of their ability to describe the inverse relation between the axial 3c4e bonds of the oligosilanylsilatrane unit and the conservation of their overall order. The nature of N → Si bond in oligosilanylsilatranes is considered within orbital and quantum-topological approaches.