Quantitative Assessment of the Lewis Acidity of Silylium Ions

The Lewis acidity of several aryl-substituted tetrylium ions was classified experimentally by applying the Gutmann–Beckett method and computationally by calculation of fluoride ion affinities (FIA) (tetrel elements = Si, Ge). According to these measures, tetrylium ions are significantly more Lewis acidic than boranes, and aryl-substituted silylium borates are among the strongest isolable Lewis acids. A fine-tuning of the Lewis acidity of silylium ions is possible by taking advantage of electronic and/or steric substituent effects.