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 [(CH 3 ) 2 CF] + AsF 6 -, [( m -CF 3 C 6 H 4 )(C 6 H 5 )CF] + AsF 6 -, and [( m -CF 3 C 6 H 4 )(C 6 H 5 )CF] + As 2 F 11 - and their comparison with that of the [( o -ClC 6 H 4 )(C 6 H 5 )CCl] + cation show that, in accord with previous theoretical calculations, chlorine stabilizes carbenium ions more efficiently than fluorine. The 13 C and 19 F NMR spectra of the [( m -CF 3 C 6 H 4 )(C 6 H 5 )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 [F 5 As−F−AsF 5 ] - anion in [( m -CF 3 C 6 H 4 )(C 6 H 5 )CF] + As 2 F 11 - possesses a symmetric fluorine bridge with an As−F−As angle of 156.5(13)° and staggered AsF 4 groups.