THE SILYLIUM ION (R(3)SI(+)) PROBLEM - EFFECT OF ALKYL SUBSTITUENTS-R

The X-ray structures of four R(3)Si(Br-6-CB11H6) species have been determined (R(3) = Et(3), i-Pr-3, t-Bu(3), and t-Bu(2)Me). Br-6-C8(11)H(6) is a particularly weakly coordinating anion and leads to long Si-Br bond distances (ca. 2.43-2.48 Angstrom). The developing silylium ion character in these species is reflected in the approach of the R(3)Si(delta+) moiety toward planarity. The sum of the C-Si-C angles for each of the four derivatives lies in the range ca. 345-351 degrees. The closest approach to 360 degrees is found in the triisopropyl derivative, not the tri-tert-butyl derivative as might be anticipated from steric considerations. One of the isopropyl groups in the i-Pr-3 derivative has its carbon atoms very nearly planar with the silicon atom. This is indicative of C-H bond hyperconjugative stabilization of the developing positive charge on silicon. An ''inorganic'' description would be in terms of an alpha-agostic C-H interaction with the electron-deficient 3p(z) orbital on silicon. The results suggest that hyperconjugative stabilization zof cationic character, although relatively unimportant compared to that for carbon in carbenium ions, is nevertheless not negligible for silicon. Solid state CPMAS Si-29 NMR data are also reported. The chemical shifts lie in the 105-115 ppm downfield region as expected for silylium ion-like character in these R(3)Si(delta+)(Br-6-CB11H6)(delta-) species.