Structure of the fluorosulfite anion: rotational disorder of SO(2)F(-) in the alkali metal fluorosulfites and crystal structures of alpha- and beta-CsSO(2)F.

19F solid-state NMR spectra of the alkali metal fluorosulfites KSO(2)F, RbSO(2)F, alpha-CsSO(2)F, and beta-CsSO(2)F show that the fluorosulfite anion is subjected to dynamical disorder at room temperature. This disorder can be modeled by 120 degrees rotational jumps with respect to the C(3)-pseudoaxis of the anion. The exchange frequency of this disorder decreases with decreasing temperature. The calculated jump frequencies at room temperature are 5 x 10(5) Hz for KSO(2)F, 2 x 10(5) Hz for RbSO(2)F, 1 x 10(7) Hz for alpha-CsSO(2)F, and 5 x 10(5) Hz for beta-CsSO(2)F, respectively. The crystal structures of alpha- and beta-CsSO(2)F were determined for the first time alpha: Pnma; Z = 4; a = 790.98(8, b = 666.07(7), c = 798.93(9) pm; T = 293 K. beta: R3m; Z = 3; a = 659.22(6), c = 800.50(4) pm; T = 293 K). X-ray single-crystal data of KSO(2)F, RbSO(2)F, and alpha-CsSO(2)F taken at various temperatures were refined using the disorder model developed from the NMR results. It is shown that it is impossible to obtain reliable structural parameters of the SO(2)F(-) ion by refinement of X-ray data as long as rotational disorder persists. The preference of an ordered structure increases with decreasing temperature. The degree of disorder remaining in alpha-CsSO(2)F at 93 K is small. The structural parameters of the fluorosulfite anion as obtained by a routine refinement of this phase (S-F, 169.0(2) pm; S-O, 147.8(1) pm) are the best lower (S-F) and upper (S-O) limits of the "true" bond lengths determined experimentally. They comply with the values obtained from quantum chemical calculations.