TOWARD METAL-STABILIZED SILYLIUM CATIONS - AN EHMO STUDY OF [(HC C-SIH2)CO-2(CO)6](+) AND X-RAY CRYSTAL-STRUCTURES OF (ME(3)SIC CSIPH(2)H)MO-2(CO)(4)CP(2) AND [(ME(3)SIC CSIME(2))CO-2(CO)(6)]O-2

Extended Huckel molecular orbital calculations on the silicon cation [(HC=C-SiH2)Co-2(CO)(6)](+) reveal that the favored geometry for this silylium ion requires the SiH2 group to lean toward a Co(CO)(3) vertex. This parallels the known behavior of carbocations with metal cluster vertices whereby the electron-deficient center is stabilized by a direct interaction with an organometallic fragment. These calculations also show that replacement of the Co(CO)(3) vertices by isolobal CpMo(CO)(2) groups noticeably enhances the stabilization of the silylium moiety. A number of possible precursors to metal-stabilized silylium ions have been synthesized, and the clusters (Me(3)SiC=C-SiPh(2)H)Mo-2(CO)(4)Cp(2), 9, and [(Me(3)SiC=C-SiMe(2))Co-2(CO)(6)]O-2, 15, have been characterized by x-ray crystallography. 9 crystallizes in the monoclinic space group P2(1)/c with a = 17.061(2) Angstrom, b = 9.756(2) Angstrom, c = 17.922(3) Angstrom, beta = 96.64(3)3 degrees, V = 2963.1(8) Angstrom(3), and Z = 4. 15 crystallizes in the monoclinic space group C2/c with a = 14.093(2) Angstrom, b = 10.285(1) Angstrom, c = 27.689(3) Angstrom, beta = 92.153(9)degrees, V = 4010.6(5) Angstrom(3), and Z = 4.