The X-ray structures of four R~S~( B I -~-C B~~H~) species have been determined (R3 = Et3, i-Pr3, t-Bu3, and t-BuzMe). Br6-CBllH6-is a particularly weakly coordinating anion and leads to long Si-Br bond distances (ca. 2.43-2.48 A). The developing silylium ion character in these species is reflected in the approach of the R3Si6+ moiety toward planarity. The sum of the C-Si-C angles for each of the four derivatives lies in the range ca. 345-351". The closest approach to 360" 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-Pr3 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 a n a-agostic C-H interaction with the electron-deficient 3pz orbital on silicon. The results suggest that hyperconjugative stabilization of cationic character, although relatively unimportant compared to that for carbon in carbenium ions, is nevertheless not negligible for silicon. Solid state CPMAS 29Si 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 R3sid+(Br6-cB~~H6)+ species. (14) Xie, Z.; Liston, D. J.; Jelinek, T.; Mitro, V.; Bau, R.; Reed, C. A. (15) Kira, M.; Hino, T.; Sakurai, H.

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The first example of an X-ray crystallographically characterized two-coordinate phosphorus cation with a P-C single o bond is described. [P(mesityl)(N-i-Pr2>][AlClJ was prepared by treatment ofPCl(mesityl)(N-i-Pr2) with AhCle in dichloromethane. The phosphenium ion character of the cation is reflected in the large downfield 31P chemical shift (500 ppm) and can be compared to 313 ppm in the corresponding bis-(dialkylamido) cation [P(N-i-Pr2)zI+.

Organometallic Compounds of the Lanthanides. 59.1 (1,1'-(3-Oxapentamethylene)dicyclopentadienyl)yttrium and -lutetium 3,5-Dimethylpyrazolates. X-ray Crystal Structure of [0(CH2CH2C5H4)2Ln(y-N2C3HMe2)(M-0H)Ln(C5H4CH2CH2)20] (Ln = Y, Lu)