Crystal structure of sodium thio-sulfate dihydrate and comparison to the penta-hydrate.

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 P 2 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 thio-sulfate anions and water mol-ecules and the anions act as mono- and bidentate ligands. In the extended structure, the thio-sulfate anions and water mol-ecules 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.