Donor-Stabilized Silyl Cations. 9. Two Dissociation Patterns of Hexacoordinate Silicon Complexes: A Model Nucleophilic Substitution at Pentacoordinate Silicon¹

Abstract

Hexacoordinate silicon bischelates with bulky monodentate alkyl ligands tend to ionize, while those with electron-withdrawing CF3 substituents on the chelate rings resist ionization. A hexacoordinate silicon complex with both of these structural features was prepared and was found, by its temperature-dependent 29Si NMR spectroscopy, to undergo a reversible neutral dissociation of the N→Si dative bond, which satisfies both tendencies. The results are supported by crystallographic structure analyses and by nonempirical MO calculations of isodesmic reaction energies.