Closely Approaching the Silylium Ion (R3Si+)

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Science  15 Oct 1993:
Vol. 262, Issue 5132, pp. 402-404
DOI: 10.1126/science.262.5132.402


The crystal structure of the tri-isopropyl silyl species, i-Pr3Si(Br6-CB11H6), where the brominated carborane Br6-CB11 H6 is perhaps the least nucleophilic anion presently known, has revealed the highest degree of silylium cation character (R3Si+) yet observed. The average C—Si—C angle is 117°, only 3° short of the planarity expected of a pure silylium ion(120°). This value compares to 114° recently reported for a toluene-solvated silyl cation, [Et3Si(toluene)]+ by Lambert and co-workers. The greater silylium ion character of i-Pr3Si(Br6-CB11H6) versus [i-Pr3Si(toluene)]+ is also reflected in the larger downfield shift of the silicon-29 nuclear magnetic resonance, 109.8 versus ≤94.0 parts per million. The unusual bonding of toluene to R3Si+ in Lambert's compound is reinterpreted as a significant covalent π interaction.