Silylium ion

 ( Cations ) 

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 . They can be viewed as protonated . Early efforts to generate these cations produced salts of the pyridine complex (CH₃)₃Si-NC₅H₅⁺, the hydride-bridged species (Et₃Si)₂H⁺, and the toluene complex (mes)₃Si(toluene)⁺.

Well-characterized silylium salts with well-defined three-coordinate silicon cations trimesitylsilylium and tris(pentamethylphenyl) . These cations are related to trityl (), 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 HCB₁₁Me₅Br₆⁻ and decaborate B₁₂Cl₁₂^2-, respectively. Weakly coordinating anions are essential for the isolation of these highly electrophilic cations.

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See also

• Silanide

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