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A silylium ion is a reactive silyl-containing cation with the formula SiR+
3. 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 silylenes. A well-characterized example is trimesitylsilylium, Si(C
6H
2Me
3)+
3. This salt has been crystallized with an anionic carborane, a weakly coordinating anion. This cation is related to trityl (C(C
6H
5)+
3), 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". In contrast, Me3SiOTf, normally considered a source of electrophilic silicon, has a 29Si NMR shift of 43 ppm.