Organometallic Chemistry. 17: ²⁹Si and ¹³C NMR Spectroscopic Study of Phenylsilyl Anions . The Question of Si–C pπ–pπ-Electron Delocalization and Comparison with Related Carbanions

A series of phenylsilyl anions, including Ph₃SiLi, Ph₂MeSiLi, PhMe₂SiLi, and Me₃SiK, has been studied by ¹³C and ²⁹Si NMR spectroscopy with particular emphasis to determine the effective delocalization of charge away from the silicon atom. The ¹³C and ²⁹Si data are discussed in this regard and evaluated as indicative of the trend of the delocalization of charge. As compared to the corresponding carbanions, the delocalization is reduced by a factor of about 10 for Ph₃SiLi and 4 for Ph₂MeSiLi and PhMe₂SiLi, indicating the substantially decreased degree of Si–C pπ–pπ interaction and thus Si=C character, as compared to carbanions showing strong C–C pπ–pπ interaction and thus double-bonded quinoidal character.