Gold compounds

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Gold(III) chloride solution in water

Gold compounds are compounds by the element

thiolates, and organophosphines. Au(I) compounds are typically linear. A good example is Au(CN)−2, which is the soluble form of gold encountered in mining. The binary gold halides, such as AuCl, form zigzag polymeric chains, again featuring linear coordination at Au. Most drugs based on gold are Au(I) derivatives.[3]

Au(III) (referred to as the auric) is a common oxidation state, and is illustrated by

covalent and ionic character. Gold(I,III) chloride is also known, an example of a mixed-valence complex
.

Gold does not react with oxygen at any temperature[4] and, up to 100 °C, is resistant to attack from ozone.[5]

Some free

gold(I) iodide
AuI.

Gold does not react with sulfur directly,[9] but gold(III) sulfide can be made by passing hydrogen sulfide through a dilute solution of gold(III) chloride or chloroauric acid.

Gold readily dissolves in mercury at room temperature to form an amalgam, and forms alloys with many other metals at higher temperatures. These alloys can be produced to modify the hardness and other metallurgical properties, to control melting point or to create exotic colors.[10]

Gold is unaffected by most acids. It does not react with

hydriodic, sulfuric, or nitric acid. It does react with selenic acid, and is dissolved by aqua regia, a 1:3 mixture of nitric acid and hydrochloric acid. Nitric acid oxidizes the metal to +3 ions, but only in minute amounts, typically undetectable in the pure acid because of the chemical equilibrium of the reaction. However, the ions are removed from the equilibrium by hydrochloric acid, forming AuCl−4 ions, or chloroauric acid
, thereby enabling further oxidation.

Gold is similarly unaffected by most bases. It does not react with

molten sodium or potassium hydroxide. It does however, react with sodium or potassium cyanide under alkaline conditions when oxygen is present to form soluble complexes.[9]

Common

oxidized
and dissolves, allowing the gold to be displaced from solution and be recovered as a solid precipitate.

Rare oxidation states

Less common oxidation states of gold include −1, +2, and +5.

The −1 oxidation state occurs in aurides, compounds containing the Au−

anion. Caesium auride (CsAu), for example, crystallizes in the caesium chloride motif;[11] rubidium, potassium, and tetramethylammonium aurides are also known.[12] Gold has the highest electron affinity of any metal, at 222.8 kJ/mol, making Au− a stable species,[13] analogous to the halides
.

Gold also has a –1 oxidation state in covalent complexes with the

dimers in a manner similar to titanium(IV) hydride.[14]

Gold(II) compounds are usually

mercury(I) ion, Hg2+2.[15][16] A gold(II) complex, the tetraxenonogold(II) cation, which contains xenon as a ligand, occurs in [AuXe4](Sb2F11)2.[17]

Gold pentafluoride, along with its derivative anion, AuF−6, and its difluorine complex, gold heptafluoride, is the sole example of gold(V), the highest verified oxidation state.[18]

Some gold compounds exhibit aurophilic bonding, which describes the tendency of gold ions to interact at distances that are too long to be a conventional Au–Au bond but shorter than van der Waals bonding. The interaction is estimated to be comparable in strength to that of a hydrogen bond.

Well-defined cluster compounds are numerous.[12] In some cases, gold has a fractional oxidation state. A representative example is the octahedral species {Au(P(C6H5)3)}2+6.

See also

References

  1. S2CID 4334587
    .
  2. doi:10.1103/PhysRevB.6.4370
    .
  3. PMID 11749494
    .
  4. UC Davis
    . 2 October 2013. Retrieved 1 May 2016.
  5. ISBN 978-0-87170-518-1
    .
  6. ISBN 978-0-12-352651-9
    .
  7. ISBN 978-0-12-352651-9
    .
  8. ^ Wiberg, Wiberg & Holleman 2001, pp. 1286–1287
  9. ^ a b Emery, J. F.; Ledditcotte, G. W. (May 1961). "Nuclear Science Series (NAS-NS 3036) The Radio Chemistry of Gold" (PDF). Oak Ridge, TN: National Academy of Sciences — National Research Council — Subcommittee on Radio Chemistry. US Atomic Energy Commission. Archived (PDF) from the original on 10 November 2004. Retrieved 24 February 2021.
  10. ^ Jewellery Alloys. World Gold Council
  11. doi:10.1016/j.solidstatesciences.2005.06.015
    .
  12. ^
    ISBN 978-0-12-352651-9
    .
  13. PMID 18762832
    .
  14. PMID 21225974
    .
  15. doi:10.1002/1521-3749(200109)627:9<2112::AID-ZAAC2112>3.0.CO;2-2
    .
  16. ISBN 978-0-85404-366-8
    .
  17. PMID 11021792
    .
  18. PMID 16639770
    .
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