Structural, Chemical, and Theoretical Evidence for the Electrophilicity of the [C6F5Xe]+ Cation in [C6F5Xe][AsF6]

[C(6)F(5)Xe][AsF(6)] was prepared by metathesis from [C(6)F(5)Xe][(C(6)F(5))(2)BF(2)]. The thermal stability of the melt (</=125 degrees C) is surprisingly high. The decomposition products reveal the ability of the cation to effect electrophilic pentafluorophenylation. [C(6)F(5)Xe][AsF(6)] crystallizes in the triclinic system, space group P, with four molecules in the unit cell. Of these, two are symmetry independent with Xe-C distances of 2.079(6) and 2.082(5) Å, Xe-F distances (cation-anion contacts) of 2.714(5) and 2.672(5) Å, and C-Xe-F angles of 170.5(3) and 174.2(3) degrees, respectively. The relation between cations and anions is best described as an asymmetric hypervalent (3c-4e) bond. Temperature dependent (19)F NMR measurements reveal the occurrence of separated ions in solution, with [C(6)F(5)Xe](+) coordinated by a basic solvent molecule. Minimum energy geometries and charge distributions were calculated for [C(6)F(5)Xe](+), [C(6)H(5)Xe](+), [C(6)F(5)](+), [C(6)H(5)](+), [CF(3)Xe](+), [CH(3)Xe](+), [C(6)F(5)Ng](+) (Ng = Kr, Ar, Ne, He), and [C(6)F(5)Xe][AsF(6)] at the ab initio RHF/LANL2DZ level. According to these calculations, C-Ng cations with short C-Ng distances are stable when the natural charge of the noble gas carries the main part of the positive net-charge and the ipso-C atom is not positive. In [C(6)F(5)Xe](+), for example, 89% of the positive charge is concentrated on Xe.