Donor-stabilized silyl cations. 9. Two dissociation patterns of hexacoordinate silicon complexes: A model nucleophilic substitution at pentacoordinate silicon

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 Si-29 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.