Atransition metal thiosulfate complex is acoordination complex containing one or morethiosulfate ligands. Thiosulfate occurs in nature and is used industrially, so its interactions with metal ions are of some practical interest.[1]
Thiosulfate is a potent ligand forsoft 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.[2] Three binding modes are common: monodentate (κ1-),O,S-bidentate (κ2-), andbridging (μ-).
Linkage isomerism (O vs S) has been observed in[Co(NH3)5(S2O3)]+.[3]
Typically, thiosulfate complexes are prepared from thiosulfate salts by displacement of aquo or chloro ligands.[2] In some cases, they arise by oxidation of polysulfido complexes, or by binding of sulfur trioxide to sulfido ligands.[4][5]
Silver-thiosulfate complexes are produced by commonphotographic fixers. By dissolvingsilver halides, the fixer stabilises the image. The dissolution process entails reactions involving the formation of 1:2 and 1:3 complexes (X = halide):[6] Fixation involves these chemical reactions (X =halide, typicallyBr−):[7]
Sodium thiosulfate andammonium thiosulfate have been proposed as alternativelixiviants tocyanide for extraction ofgold from ores[8] and printed circuit boards.[9] The complex[Au(S2O3)2]3- is assumed to be the principal product in such extractions. Presentlycyanide salts are used on a large scale for that purpose with obvious risks.[8] The advantages of this approach are that (i) thiosulfate is far less toxic than cyanide and (ii) that ore types that arerefractory togold cyanidation (e.g.carbonaceous orCarlin-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 notadsorb toactivated carbon, which is the standard technique used in gold cyanidation to separate the gold complex from the ore slurry.[8]
In theIUPAC 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".[10] Thus, sodium aurothiosulfate could be called trisodium di(thiosulfato)aurate(I).