Structures of the BrF₄⁺ and IF₄⁺ Cations

The large discrepancies between the calculated and observed structures for BrF4+ and IF4+ (Christe, K. O.; Zhang, X.; Sheehy, J. A.; Bau, R. J. Am. Chem. Soc. 2001, 123, 6338) prompted a redetermination of the crystal structures of BrF4+Sb2F11- (monoclinic, P21/c, a = 5.2289(6) Å, b = 14.510(2) Å, c = 14.194(2) Å, β = 90.280(1)°, Z = 4) and IF4+SbF6- (orthorhombic, Ibca, a = 8.2702(9) Å, b = 8.3115(9) Å, c = 20.607(2) Å, Z = 8). It is shown that for BrF4+, the large differences were mainly due to large errors in the original experimental data. For IF4+SbF6-, the geometry previously reported for IF4+ was reasonably close to that found in this study despite a very large R-factor of 0.15 and a refinement in an incorrect space group. The general agreement between the calculated and the redetermined geometries of BrF4+ and IF4+ is excellent, except for the preferential compression of one bond angle in each ion due to the influence of interionic fluorine bridges. In BrF4+, the fluorine bridges are equatorial and compress this angle. In IF4+, the nature of the fluorine bridges depends on the counterion, and either the axial (in IF4+SbF6-) or the equatorial (in IF4+Sb2F11-) bond angle is preferentially compressed. Therefore, the geometries of the free ions are best described by the theoretical calculations.