Structural, chemical, and theoretical evidence for the electrophilicity of the [C6F5Xe]+ cation in [C6F5Xe][AsF6]

[C6F5Xe][AsF6] was prepared by metathesis from [C6F5Xe][(C6F5)(2)BF2]. The thermal stability of the melt (less than or equal to 125 degrees C) is surprisingly high. The decomposition products reveal the ability of the cation to effect electrophilic pentafluorophenylation. [C6F5Xe][AsF6] crystallizes in the triclinic system, space group P (1) over bar, 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) Angstrom, Xe-F distances (cation-anion contacts) of 2.714(5) and 7.672(5) Angstrom, 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 F-19 NMR measurements reveal the occurrence of separated ions in solution, with [C6F5Xe](+) coordinated by a basic solvent molecule. Minimum energy geometries and charge distributions were calculated for [C6F5Xe](+), [C6H5Xe](+), [C6F5](+), [C6H5](+), [CF3Xe](+), [CH3Xe](+), [C(6)F(5)Ng](+) (Ng = Kr, Ar, Ne, He), and [C6F5Xe][AsF6] 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 [C6F5Xe](+), for example, 89% of the positive charge is concentrated on Xe.