Crystal structure of sodium thiosulfate dihydrate and comparison to the pentahydrate

Na2S2O3·2H2O has been mentioned in the literature for more than a hundred years and pure samples were prepared and investigated, however, no structural data except for a set of lattice parameters were known to date. Now crystals of this compound have been grown at the surface of an aqueous solution of Na2S2O3 and the structure has been determined at 200 and 100 K. Na2S2O3·2H2O crystallizes in the space group P21/n with two formula units in the asymmetric unit and all atoms occupying general positions. The sodium cations are five- to seven-coordinate by thiosulfate anions and water molecules and the anions act as mono- and bidentate ligands. In the extended structure, the thiosulfate anions and water molecules are connected by O—H...O and O—H...S hydrogen bonds of medium strength to form corrugated layers, which are linked by sodium cations. For comparison, the crystal structure of Na2S2O3·5H2O has been determined at the same conditions, i.e. for the first time below room temperature.