Binuclear Ethylene-Bridged Silicon Chelates: Equilibrium between Neutral Hexacoordinate and Ionic Pentacoordinate Siliconium Complexes

Binuclear hexacoordinate silicon complexes with two N→Si dative bonds (per Si atom) have been prepared. The crystal structure conforms to a distorted octahedron, with the N→Si bonds trans to each other. The binuclear complex undergoes Si-Cl dissociation of one Si–Cl bond in CD₂Cl₂ and CDCl₃ solution, to form a binuclear, monosiliconium chloride salt, in reversible equilibrium with its precursor. The dissociation and equilibrium reactions are observed by variable-temperature ²⁹Si NMR spectra. The extent of ionic dissociation increases as the temperature is decreased. The equilibrium population ratio is shifted completely to the di-ionic side at room temperature, by replacement of the chloride by the less nucleophilic triflate anion. The crystal structure of a disiliconium ditriflate shows a) well separated ions, b) that the geometry about silicon is almost an exact square pyramid, and c) that the N→Si bonds are among the shortest coordination bonds of this kind.