Transition metal thiosulfate complex

A transition metal thiosulfate complex is a coordination complex containing one or more thiosulfate ligands. Thiosulfate occurs in nature and is used industrially, so its interactions with metal ions are of some practical interest.

Examples
Thiosulfate is a potent ligand for soft metal ions. A typical complex is [Pd(S2O3)2(ethylenediamine)](2-), which features a pair of S-bonded thiosulfate ligands. Simple aquo and ammine complexes are also known. Three binding modes are common: monodentate (κ1-), O,S-bidentate (κ2-),and bridging (μ-).

Linkage isomerism (O vs S) has been observed in [Co(NH3)5(S2O3)]+.

Preparation
Typically, thiosulfate complexes are prepared from thiosulfate salts by displacement of aquo or chloro ligands. In some cases, they arise by oxidation of polysulfido complexes, or by binding of sulfur trioxide to sulfido ligands.

Photography
Silver-thiosulfate complexes are produced by common photographic fixers. By dissolving silver halides, the fixer stabilises the image. The dissolution process entails reactions involving the formation of 1:2 and 1:3 complexes (X = halide): Fixation involves these chemical reactions (X = halide, typically Br−):
 * AgX + 2 S2O3(2−) → [Ag(S2O3)2](3−) + X−
 * AgX + 3 S2O3(2−) → [Ag(S2O3)3](5−) + X−

Recovery of precious metals
Sodium thiosulfate and ammonium thiosulfate have been proposed as alternative lixiviants to cyanide for extraction of gold from ores and printed circuit boards. The complex [Au(S2O3)2]3- is assumed to be the principal product in such extractions. Presently cyanide salts are used on a large scale for that purpose with obvious risks. The advantages of this approach are that (i) thiosulfate is far less toxic than cyanide and (ii) that ore types that are refractory to gold cyanidation (e.g. carbonaceous or Carlin-type ores) can be leached by thiosulfate. One problem with this alternative process is the high consumption of thiosulfate, which is more expensive than cyanide. Another issue is the lack of a suitable recovery technique since [Au(S2O3)2](3−) does not adsorb to activated carbon, which is the standard technique used in gold cyanidation to separate the gold complex from the ore slurry.

Naming
In the IUPAC Red Book the following terms may be used for thiosulfate as a ligand: trioxido-1κ3O-disulfato(S—S)(2−); trioxidosulfidosulfato(2−); thiosulfato; sulfurothioato. In the naming for thiosulfate salts, the final "o" is replaced by "e". Thus, sodium aurothiosulfate could be called trisodium di(thiosulfato)aurate(I).