A transition metal thiosulfate complex is a

coordination complex A coordination complex is a chemical compound consisting of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of chemical bond, bound molecules or ions, that are in turn known as ' ...

containing one or more

thiosulfate Thiosulfate ( IUPAC-recommended spelling; sometimes thiosulphate in British English) is an oxyanion of sulfur with the chemical formula . Thiosulfate also refers to the compounds containing this anion, which are the salts of thiosulfuric acid, ...

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 , 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 .

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.

Applications

Photography

Silver-thiosulfate complexes are produced by common

photographic fixer Photographic fixer is a mixture of chemicals used in the final step in the photographic processing of film or paper. The fixer stabilises the image, removing the unexposed silver halide remaining on the photographic film or photographic paper, le ...

s. By dissolving

silver halide A silver halide (or silver salt) is one of the chemical compounds that can form between the Chemical element, element silver (Ag) and one of the halogens. In particular, bromine (Br), chlorine (Cl), iodine (I) and fluorine (F) may each combine wit ...

s, 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 In chemistry, a halide (rarely halogenide) is a binary chemical compound, of which one part is a halogen atom and the other part is an element or radical that is less electronegative (or more electropositive) than the halogen, to make a fl ...

, typically ):Karlheinz Keller et al. "Photography" in Ullmann's Encyclopedia of Industrial Chemistry, 2005, Wiley-VCH, Weinheim. : :

Recovery of precious metals

Sodium thiosulfate Sodium thiosulfate (sodium thiosulphate) is an inorganic compound with the formula . Typically it is available as the white or colorless pentahydrate (x = 5), which is a white solid that dissolves well in water. The compound is a reducing agent an ...

and

ammonium thiosulfate Ammonium thiosulfate (ammonium thiosulphate in British English) is an inorganic compound with the formula . It is white crystalline solid with ammonia odor, readily soluble in water, slightly soluble in acetone and insoluble in ethanol and diethy ...

have been proposed as alternative lixiviants to

cyanide In chemistry, cyanide () is an inorganic chemical compound that contains a functional group. This group, known as the cyano group, consists of a carbon atom triple-bonded to a nitrogen atom. Ionic cyanides contain the cyanide anion . This a ...

for extraction of

gold Gold is a chemical element; it has chemical symbol Au (from Latin ) and atomic number 79. In its pure form, it is a brightness, bright, slightly orange-yellow, dense, soft, malleable, and ductile metal. Chemically, gold is a transition metal ...

from ores and printed circuit boards. The complex u(S₂O₃)₂sup>3- is assumed to be the principal product in such extractions. Presently

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 In materials science, a refractory (or refractory material) is a material that is resistant to decomposition by heat or chemical attack and that retains its strength and rigidity at high temperatures. They are inorganic, non-metallic compound ...

gold cyanidation Gold cyanidation (also known as the cyanide process or the MacArthur–Forrest process) is a hydrometallurgical technique for extracting gold from low-grade ore through conversion to a water-soluble coordination complex. It is the most commonly ...

(e.g.

carbon Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...

aceous 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 does not

adsorb Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the ''adsorbate'' on the surface of the ''adsorbent''. This process differs from absorption, in which ...

activated carbon Activated carbon, also called activated charcoal, is a form of carbon commonly used to filter contaminants from water and air, among many other uses. It is processed (activated) to have small, low-volume pores that greatly increase the surface ar ...

, which is the standard technique used in gold cyanidation to separate the gold complex from the ore slurry.

Naming

In the

IUPAC Red Book Nomenclature of Inorganic Chemistry, IUPAC Recommendations 2005 is the 2005 version of ''Nomenclature of Inorganic Chemistry'' (which is informally called the Red Book). It is a collection of rules for naming inorganic compounds, as recommended by ...

the following terms may be used for thiosulfate as a ligand: trioxido-1κ³''O''-disulfato(''S''—''S'')(2−); trioxidosulfidosulfato(2−); thiosulfato; sulfurothioato. In the naming for thiosulfate salts, the final "o" is replaced by "e".IUPAC Red Book p 329 Thus, sodium aurothiosulfate could be called trisodium di(thiosulfato)aurate(I).

References

{{Coordination complexes Coordination chemistry Coordination complexes Ligands Thiosulfates