Crystal structure of sodium thio­sulfate dihydrate and comparison to the penta­hydrate

Na(2)S(2)O(3)·2H(2)O 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 Na(2)S(2)O(3) and the structure has been determined at 200 and 100 K. Na(2)S(2)O(3)·2H(2)O crystallizes in the space group P2(1)/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 Na(2)S(2)O(3)·5H(2)O has been determined at the same conditions, i.e. for the first time below room temperature.