Relative abilities of fluorine and chlorine to stabilize carbenium ions:: Crystal structures of two fluoro-substituted carbocations and of As2F11-

The first crystal structures of fluoro-substituted carbocations and of the AS(2)F(11)(-) anion are reported. The experimental geometries of the carbenium ions in [(CH3)(2)CF]+AsF6-, [(m-CF3C6H4) (C6H5)CF](+) AsF6-, and [(m-CF3C6H4)(C6H5)CF](+)AS(2)F(11)(-) and their comparison with that of the [(o-ClC6H4)(C6H5)CCl](+) cation show that, in accord with previous theoretical calculations, chlorine stabilizes carbenium ions more efficiently than fluorine. The C-13 and F-19 NMR spectra of the [(m-CF3C6H4)(C6H5)CF](+) cation were recorded and analyzed with the help of RHF/6-31G(d,p) calculations using the GIAO method. In each of the three fluoro-substituted carbocation crystal structures studied, the carbenium centers are further stabilized by forming two fluorine bridges with the anions, resulting in distorted trigonal bipyramidal environments around the carbenium centers. The [F5As-F-AsF5](-) anion in [(m-CF3C6H4)(C6H5)CF]+As2F11- possesses a symmetric fluorine bridge with an As-F-As angle of 156.5(13)degrees and staggered AsF4 groups.