Sodium aurothiosulfate

Sodium aurothiosulfate, or sanocrysin, is the inorganic compound with the formula Na3[Au(S2O3)2]*2H2O. It is the trisodium salt of the coordination complex of gold(I), [Au(S2O3)2](3-). The dihydrate, which is colorless, crystallizes with two waters of crystallization. The compound has some medicinal properties as well as potential for hydrometallurgy.

Structure
The anionic complex features a linear AuS2 core and is overall centrosymmetric. Like most other thiosulfate complexes, only the planetary sulfur of thiosulfate is coordinated to the metal.

History
The salt is typically prepared by reduction of gold(III) chloride with thiosulfate:
 * 4 Na2S2O3 + AuCl3  ->   Na3[Au(S2O3)2]  +  Na2S4O6  +  3 NaCl

The compound was first synthesized in 1845 by Mathurin-Joseph Fordos and A. Gélis who were researching chemicals used in the Daguerrotype photographic process. It then came to be called Fordos and Gélis salt. It went out of interest until 1924 when it was noted as a chemotherapeutic agent for tuberculosis by Holger Møllgaard in Copenhagen. Other methods of synthesis were then identified.

Potential applications
Like several other gold compounds, this species is used as an antirheumatic. The first placebo-controlled trial was probably conducted in 1931, when sanocrysin was compared with distilled water for the treatment of tuberculosis.

Aurothiosulfate complexes have also been discussed in the context of the extraction of gold from its ores. The general approach would employ sodium or ammonium thiosulfate in place of cyanide salts as lixiviants.