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

The X-ray structures of four R(3)Si(Br-6-CB11H6) species have been det ermined (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 c haracter in these species is reflected in the approach of the R(3)Si(d elta+) 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 clo sest 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 h as its carbon atoms very nearly planar with the silicon atom. This is indicative of C-H bond hyperconjugative stabilization of the developin g 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 s tabilization zof cationic character, although relatively unimportant c ompared to that for carbon in carbenium ions, is nevertheless not negl igible for silicon. Solid state CPMAS Si-29 NMR data are also reported . The chemical shifts lie in the 105-115 ppm downfield region as expec ted for silylium ion-like character in these R(3)Si(delta+)(Br-6-CB11H 6)(delta-) species.