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[[File:Gold(III) chloride solution.jpg|thumb|right|Gold(III) chloride solution in water]]
'''Gold compounds''' are compounds by the element [[gold]] (Au). Although gold is the most noble of the [[noble metal]]s,<ref>{{cite journal |doi=10.1038/376238a0 |title=Why gold is the noblest of all the metals |date=1995 |last1=Hammer |first1=B. |last2=Norskov |first2=J. K. |journal=Nature |volume=376 |issue=6537 |pages=238–240 |bibcode=1995Natur.376..238H|s2cid=4334587 }}</ref><ref>{{cite journal |doi=10.1103/PhysRevB.6.4370 |title=Optical Constants of the Noble Metals |date=1972 |last1=Johnson |first1=P. B. |last2=Christy |first2=R. W. |journal=Physical Review B |volume=6 |issue=12 |pages=4370–4379 |bibcode=1972PhRvB...6.4370J}}</ref> it still forms many diverse compounds. The [[oxidation state]] of gold in its compounds ranges from −1 to +5, but Au(I) and Au(III) dominate its chemistry. Au(I), referred to as the aurous ion, is the most common oxidation state with soft [[ligand]]s such as [[thioether]]s, [[thiolate]]s, and [[organophosphine]]s. Au(I) compounds are typically linear. A good example is {{chem2|[[gold cyanidation|Au(CN)2(−)]]}}, which is the soluble form of gold encountered in mining. The binary [[gold halide]]s, such as [[gold(I) chloride|AuCl]], form zigzag polymeric chains, again featuring linear coordination at Au. Most drugs based on gold are Au(I) derivatives.<ref>{{cite journal |last=Shaw III |first=C. F. |title=Gold-Based Medicinal Agents |journal=Chemical Reviews |date=1999 |volume=99 |issue=9 |pages=2589–2600 |doi=10.1021/cr980431o |pmid=11749494}}</ref>
Au(III) (referred to as the auric) is a common oxidation state, and is illustrated by [[gold(III) chloride]], {{chem2|Au2Cl6}}. The gold atom centers in Au(III) complexes, like other d<sup>8</sup> compounds, are typically [[square planar]], with [[chemical bond]]s that have both [[covalent]] and [[ion]]ic character. [[Gold(I,III) chloride]] is also known, an example of a [[mixed-valence complex]].
Gold does not react with oxygen at any temperature<ref>{{cite web |url=http://chemwiki.ucdavis.edu/Core/Inorganic_Chemistry/Descriptive_Chemistry/Elements_Organized_by_Block/2_p-Block_Elements/Group_16%253A_The_Oxygen_Family/Chemistry_of_Oxygen |title=Chemistry of Oxygen |website=Chemwiki [[UC Davis]]|access-date=1 May 2016 |date=2 October 2013}}</ref> and, up to 100 °C, is resistant to attack from ozone.<ref>{{cite book |editor-last1=Craig |editor-first1=B. D.|editor-last2= Anderson|editor-first2=D. B. |title=Handbook of Corrosion Data |date=1995 |publisher=ASM International |location=Materials Park, Ohio |isbn=978-0-87170-518-1 |page=587}}</ref>
: <math>\mathrm{Au}+\mathrm{O}_2 \neq</math>
: <math> \mathrm{Au}+\mathrm{O}_3 \overset{\underset{t<100^\circ\text{C}}{}}{\neq}</math>
Some free [[halogen]]s react with gold.<ref>{{Cite book |last1=Wiberg |first1=Egon |last2=Wiberg |first2=Nils |last3=Holleman |first3=Arnold Frederick |name-list-style=amp |date=2001 |title=Inorganic Chemistry |edition=101st |publisher=Academic Press |isbn=978-0-12-352651-9 |page=1286 }}</ref> Gold is strongly attacked by fluorine at dull-red heat<ref>{{Cite book |url=https://books.google.com/books?id=Mtth5g59dEIC |title=Inorganic Chemistry |last1=Wiberg |first1=Egon |last2=Wiberg |first2=Nils |date=2001 |publisher=Academic Press |isbn=978-0-12-352651-9 |page=404}}</ref> to form [[gold(III) fluoride]] {{chem2|AuF3}}. Powdered gold reacts with chlorine at 180 °C to form [[gold(III) chloride]] {{chem2|AuCl3}}.<ref>{{harvnb|Wiberg|Wiberg|Holleman|2001|pp=1286–1287}}</ref> Gold reacts with bromine at 140 °C to form [[gold(III) bromide]] {{chem2|AuBr3}}, but reacts only very slowly with iodine to form [[gold(I) iodide]] AuI.
: <chem>2 Au + 3 F2 ->[t] 2 AuF3</chem>
: <chem>2 Au + 3 Cl2 ->[t] 2 AuCl3</chem>
: <chem>2 Au + 2 Br2 ->[t] AuBr3 + AuBr</chem>
: <chem>2 Au + I2 ->[t] 2 AuI</chem>
Gold does not react with sulfur directly,<ref name="library.lanl.gov">{{cite web |url=http://library.lanl.gov/cgi-bin/getfile?rc000062.pdf |last1=Emery |first1=J. F. |last2=Ledditcotte |first2=G. W. |title=Nuclear Science Series (NAS-NS 3036) The Radio Chemistry of Gold |date=May 1961 |agency=US Atomic Energy Commission |publisher=National Academy of Sciences — National Research Council — Subcommittee on Radio Chemistry |location=Oak Ridge, TN |url-status=live |access-date=24 February 2021 |archive-url=https://web.archive.org/web/20041110193206/http://library.lanl.gov/cgi-bin/getfile?rc000062.pdf |archive-date=10 November 2004}}</ref> but [[gold(III) sulfide]] can be made by passing [[hydrogen sulfide]] through a dilute solution of gold(III) chloride or [[chlorauric acid]].
Gold readily dissolves in [[mercury (element)|mercury]] at room temperature to form an [[amalgam (chemistry)|amalgam]], and forms [[alloy]]s 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.<ref name="utilisegold">[https://web.archive.org/web/20080619061619/http://www.utilisegold.com/jewellery_technology/colours/colour_alloys/ Jewellery Alloys]. World Gold Council</ref>
Gold is unaffected by most acids. It does not react with [[hydrofluoric acid|hydrofluoric]], [[hydrochloric acid|hydrochloric]], [[hydrobromic acid|hydrobromic]], [[hydriodic acid|hydriodic]], [[sulfuric acid|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 {{chem2|AuCl4(−)}} ions, or [[chloroauric acid]], thereby enabling further oxidation.
: <chem>2 Au + 6 H2SeO4 ->[200^\circ C] Au2(SeO4)3 + 3 H2SeO3 + 3 H2O</chem>
: <chem>Au + 4HCl + HNO3 -> H[AuCl4] {}+ NO\uparrow + 2H2O </chem>
Gold is similarly unaffected by most bases. It does not react with [[aqueous solution|aqueous]], [[solid]], or [[molten]] [[sodium hydroxide|sodium]] or [[potassium hydroxide]]. It does however, react with [[sodium cyanide|sodium]] or [[potassium cyanide]] under alkaline conditions when [[oxygen]] is present to form soluble complexes.<ref name="library.lanl.gov" />
Common [[oxidation state]]s of gold include +1 (gold(I) or aurous compounds) and +3 (gold(III) or auric compounds). Gold ions in solution are readily [[reduction (chemistry)|reduced]] and [[precipitation (chemistry)|precipitated]] as metal by adding any other metal as the [[reducing agent]]. The added metal is [[oxidation|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 {{chem2|Au−}} [[anion]]. [[Caesium auride]] (CsAu), for example, crystallizes in the [[caesium chloride]] motif;<ref name="relativist_Au_Pt">{{Cite journal |title=Effects of relativistic motion of electrons on the chemistry of gold and platinum |first=Martin |last=Jansen |journal=Solid State Sciences |volume=7 |issue=12 |date=2005 |doi=10.1016/j.solidstatesciences.2005.06.015 |pages=1464–1474 |bibcode=2005SSSci...7.1464J|doi-access=free}}</ref> rubidium, potassium, and [[tetramethylammonium]] aurides are also known.<ref name="Holleman">{{cite book |last1=Holleman |first1=A. F. |last2=Wiberg |first2=E. |title=Inorganic Chemistry |publisher=Academic Press |location=San Diego |year=2001 |isbn=978-0-12-352651-9}}</ref> Gold has the highest [[electron affinity]] of any metal, at 222.8 kJ/mol, making {{chem2|Au−}} a stable species,<ref name="martin08">{{cite journal |last=Jansen |first=Martin |title=The chemistry of gold as an anion |journal=Chemical Society Reviews |date=2008 |volume=37 |issue=9 |pages=1826–1835 |doi=10.1039/b708844m |pmid=18762832}}</ref> analogous to the [[halide]]s.
Gold also has a –1 oxidation state in covalent complexes with the [[Group 4 element|group 4]] transition metals, such as in titanium tetraauride and the analogous zirconium and hafnium compounds. These chemicals are expected to form gold-bridged [[dimer (chemistry)|dimer]]s in a manner similar to [[titanium(IV) hydride]].<ref>{{cite journal |title= Gold Behaves as Hydrogen in the Intermolecular Self-Interaction of Metal Aurides MAu<sub>4</sub> (M=Ti, Zr, and Hf) |first1= Jaehoon |last1= Jung |first2= Hyemi |last2= Kim |first3= Jong Chan |last3= Kim |first4= Min Hee |last4= Park |first5= Young-Kyu |last5= Han |journal= Chemistry: An Asian Journal |volume= 6 |issue= 3 |year= 2011 |pages= 868–872 |doi= 10.1002/asia.201000742 |pmid= 21225974 }}</ref>
Gold(II) compounds are usually [[diamagnetic]] with Au–Au bonds such as [{{chem2|Au(CH2)2P(C6H5)2]2Cl2}}. The evaporation of a solution of {{chem2|Au(OH)3}} in concentrated {{chem2|H2SO4}} produces red crystals of [[gold(II) sulfate]], {{chem2|Au2(SO4)2}}. Originally thought to be a mixed-valence compound, it has been shown to contain {{chem2|Au2(4+)}} cations, analogous to the better-known [[mercury(I)]] ion, {{chem2|Hg2(2+)}}.<ref>{{Cite journal |last=Wickleder |first=Mathias S. |doi=10.1002/1521-3749(200109)627:9<2112::AID-ZAAC2112>3.0.CO;2-2 |date=2001 |title=AuSO<sub>4</sub>: A True Gold(II) Sulfate with an Au<sub>2</sub><sup>4+</sup> Ion |journal=Journal of Inorganic and General Chemistry |volume=627 |pages=2112–2114 |issue=9}}</ref><ref>{{Cite book |last=Wickleder |first=Mathias S. |title=Handbook of chalcogen chemistry: new perspectives in sulfur, selenium and tellurium |editor-first=Francesco A. |editor-last=Devillanova |publisher=Royal Society of Chemistry |date=2007 |isbn=978-0-85404-366-8 |pages=359–361 |url=https://books.google.com/books?id=IvGnUAaSqOsC&pg=PA359}}</ref> A gold(II) complex, the [[tetraxenonogold(II)]] cation, which contains [[xenon]] as a ligand, occurs in {{chem2|[AuXe4](Sb2F11)2}}.<ref>{{Cite journal |last1=Seidel |first1=S. |last2=Seppelt |first2=K. |title=Xenon as a Complex Ligand: The Tetra Xenono Gold(II) Cation in AuXe<sub>4</sub><sup>2+</sup>(Sb<sub>2</sub>F<sub>11</sub><sup>−</sup>)<sub>2</sub> |journal=Science |date=2000 |volume=290 |issue=5489 |pages=117–118 |doi=10.1126/science.290.5489.117 |pmid=11021792 |bibcode=2000Sci...290..117S}}</ref>
[[Gold pentafluoride]], along with its derivative anion, {{chem2|AuF6-}}, and its [[difluorine complex]], [[gold heptafluoride]], is the sole example of gold(V), the highest verified oxidation state.<ref>{{Cite journal |last1=Riedel |first1=S. |last2=Kaupp |first2=M. |title=Revising the Highest Oxidation States of the 5d Elements: The Case of Iridium(+VII) |journal=Angewandte Chemie International Edition |date=2006 |volume=45 |issue=22 |pmid=16639770 |pages=3708–3711 |doi=10.1002/anie.200600274}}</ref>
Some gold compounds exhibit ''[[aurophilicity|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 force|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.<ref name="Holleman" /> In some cases, gold has a fractional oxidation state. A representative example is the octahedral species {{chem2|{Au([[triphenylphosphine|P(C6H5)3]])}6(2+)}}.
== See also ==
* [[Copper compounds]]
* [[Silver compounds]]
== References ==
{{reflist}}
{{Gold compounds}}
[[:Category:Gold compounds| ]]
[[:Category:Gold|Compounds]]
[[:Category:Chemical compounds by element]]
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