A silylium ion is a reactive silyl-containing cation with the formula . With three rather than the usual four bonds to Si, silylium ions are the silicon analogues of carbenium ions. They can be viewed as protonated

silylene Silylene is a chemical compound with the formula SiH2. It is the silicon analog of methylene, the simplest carbene. Silylene is a stable molecule as a gas but rapidly reacts in a bimolecular manner when condensed. Unlike carbenes, which can exis ...

s. Early efforts to generate these cations produced salts of the pyridine complex CH₃)₃Si-NC₅H₅sup>+, the hydride-bridged species Et₃Si)₂Hsup>+, and the toluene complex mes)₃Si(toluene)sup>+. Well-characterized silylium salts with well-defined three-coordinate silicon cations trimesitylsilylium and tris(pentamethylphenyl) . These cations are related to

trityl Triphenylmethane, or triphenyl methane, is the hydrocarbon with the formula (C6H5)3CH. This colorless solid is soluble in nonpolar organic solvents and not in water. Triphenylmethane is the basic skeleton of many synthetic dyes called triarylmet ...

(), with the extra methyl groups providing steric protection, compensating for the greater size of Si vs C. Its ²⁹Si NMR chemical shift is 225.5 ppm, downfield of TMS, which indicates that the cation is quite "naked". Trimethylsilyl trifluoromethanesulfonate (Me₃SiOTf), normally considered a source of electrophilic silicon, has a ²⁹Si NMR shift of 43 ppm. Salts of and have been crystallized with the carborane CB₁₁Me₅Br₆sup>− and decaborate ₁₂Cl₁₂sup>2-, respectively.

Weakly coordinating anion Anions that interact weakly with cations are termed non-coordinating anions, although a more accurate term is weakly coordinating anion. Non-coordinating anions are useful in studying the reactivity of electrophilic cations. They are commonly foun ...

s are essential for the isolation of these highly electrophilic cations.

References

Cations Reactive intermediates Silicon {{chem-stub