An unexpected oxidation: NaK5Cl2(S2O6)2 revisited

The title compound, NaK5Cl2(S2O6)2 [systematic name: Sodium pentapotassium dichloride bis(dithionate)], arose as an unexpected product from an organic synthesis that used dithionite (S2O4 2-) ions as a reducing agent to destroy excess permanganate ions. Compared to the previous study [Stanley (1953). Acta Cryst. 6, 187-196], the present tetragonal structure exhibits a root 2a × root 2a × c super-cell due to subtle changes in the orientations of the dithionate anions. The structure can be visualized as a three-dimensional framework of [001] columns of alternating trans-NaO4Cl2 and KO4Cl2 octahedra cross-linked by the dithionate ions with the interstices occupied by KO6Cl2 polyhedra to generate a densely packed three-dimensional framework. The asymmetric unit comprises two sodium ions (site symmetries 4 and -4, four potassium ions (site symmetries = -4, 4, 1 and 1), three chloride ions (site symmetries = 4, 4 and 2) and two half-dithionate ions (all atoms on general positions). Both dithionate ions are completed by crystallographic inversion symmetry. The crystal chosen for data collection was found to be rotationally twinned by 180° about the [100] axis in reciprocal space with a 0.6298(13):0.3702(13) domain ratio.