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 finetuning of the Lewis acidity of silylium ions is possible by taking advantage of electronic and/or steric substituent effects.

The synthesis of two series of silylated chalconium borates, 9 and 10, which are based on the peri-naphthyl and peri-acenaphthyl framework, is reported (chalcogen (Ch): O, S, Se, Te). NMR investigations of the selenium- and tellurium-containing precursor silanes 3 d-f and 8 d, f revealed a significant through-space J-coupling between the chalcogen nuclei and the Me SiH group. Experimental and computational results typify the synthesized cations 9 and 10 as chalconium ions. The imposed ring strain weakens the Si-Ch linkage compared to acyclic chalconium ions. This attenuation of the Si-Ch bond strength is more pronounced in the acenaphthene series. Surprisingly, the Si-O bonds in oxonium ions 9 a and 10 a are the weakest Si-Ch linkage in both series. The synthesized silyl chalconium borates are active in hydrodefluorination reactions of alkyl fluorides with silanes. A cooperative activation of the silane by the Lewis acidic (silicon) and by the Lewis basic side (chalcogen) is suggested.

Silylium ions undergo a single-electron reduction with phosphanes, leading to transient silyl radicals and the corresponding stable phosphoniumyl radical cations. As supported by DFT calculations, phosphanes with electron-rich 2,6-disubstituted aryl groups are sufficiently strong reductants to facilitate this single-electron transfer (SET). Frustration as found in kinetically stabilized triarylsilylium ion/phosphane Lewis pairs is not essential, and silylphosphonium ions, which are generated by conventional Lewis adduct formation of solvent-stabilized trialkylsilylium ions and phosphanes, engage in the same radical mechanism. The trityl cation, a Lewis acid with a higher electron affinity, even oxidizes trialkylphosphanes, such as tBu P, which does not react with either B(C F ) or silylium ions.

Silyliumionen unterliegen einer Einelektronenreduktion mit Phosphanen unter Bildung von kurzlebigen Silylradikalen und den entsprechenden stabilen Phosphanradikalkationen. Experimentelle Untersuchungen, unterstützt durch DFT‐Rechnungen, zeigen, dass Phosphane mit elektronenreichen 2,6‐disubstituierten Arylgruppen ausreichend starke Reduktionsmittel sind, um diese Einelektronenübertragung zu ermöglichen. Frustration, wie sie in kinetisch stabilisierten Triarylsilyliumion/Phosphan‐Lewis‐Paaren auftritt, ist nicht zwingend erforderlich. So gehen Silylphosphoniumionen, die durch konventionelle Lewis‐Adduktbildung aus lösungsmittelstabilisierten Trialkylsilyliumionen und Phosphanen gebildet werden, ebenfalls diesen radikalischen Mechanismus ein. Das Tritylkation, eine Lewis‐Säure mit höherer Elektronenaffinität, oxidiert sogar Trialkylphosphane wie tBu3P, das weder mit B(C6F5)3 noch mit Silyliumionen reagiert.

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