An X-ray crystallographic study of [Xe2F3][SbF6] and dimorphism in [Xe2F3][AsF6];: and a density functional theory study of the Xe2F3+ cation

The Xe2F3+ cation has been studied by X-ray crystallography with AsF6- and SbF6- as counter anions. The [Xe2F3][AsF6] Salt was found to be dimorphic with a monoclinic phase: C2/c, a=25.756(3) Angstrom, b=8.556(1) Angstrom, c=15.356(2) Angstrom, beta=126.592(5)degrees, V=2716.9(6) Angstrom(3), Z=12 and R-1=0.0389 at -127 degrees C; and a trigonal phase: P3(2)21, a=8.602(5) Angstrom, c=10.665(9) Angstrom, V=683.4(8) Angstrom(3), 2=3 and R-1=0.0385 at -116 degrees C. The [Xe2F3][SbF6] salt crystallizes in the monoclinic system: Clc, a=14.575(2) Angstrom, b=8.1081(12) Angstrom, c=9.926(14) Angstrom, beta=130.360(3)degrees, V=899.8(2) Angstrom(3), Z=4 and R-1=0.0319 at -127 degrees C. The Xe ... F-b... Xe bridge angle of the V-shaped Xe2F3+ cation was found to vary from 139.8(8) to 160.3(3)degrees in trigonal [Xe2F3][AsF6] and [Xe2F3][SbF6], respectively. The variation in the Xe ... F-b... Xe bridge angle is attributed to crystal packing effects. The geometry and vibrational frequencies of the Xe2F3+ cation were calculated using density functional theory methods, confirming the experimental evidence for the facile deformation of the Xe ... F-b... Xe bridge angle. The vibrational spectrum of Xe2F3+ has been reassigned based on the theoretical calculations. (C) 2000 Elsevier Science S.A. All rights reserved.