Compounds

Although gold is a noble metal, it forms many and diverse compounds. The oxidation state of gold in its compound ranges from -1 to 5+ but Au(I) and Au(III) dominate. Gold(I), referred to as the aurous ion, is the most common oxidation state with “soft” ligands such as thioethers, thiolates, and tertiary phosphines. Au(I) compounds are typically linear. A good example is Au(CN)₂^–, which is the soluble form of gold encountered in mining. Curiously, aurous complexes of water are rare. The binary Gold Halides, such as AuCl, form zig-zag polymeric chains, again featuring linear coordination at Au. Most drugs based on gold are Au(I) derivatives.^[18]

Gold(III) (“auric”) is a common oxidation state and is illustrated by gold(III) chloride, AuCl₃. Its derivative is chloroauric acid, HAuCl₄, which forms when Au dissolves in aqua regia. Au(III) complexes, like other d⁸ compounds, are typically square planar.

Less common oxidation states: Au(-I), Au(II), and Au(V)

Compounds containing the Au^- anion are called aurides. Caesium auride, CsAu which crystallizes in the caesium chloride motif. Other aurides include those of Rb⁺, K⁺, and tetramethylammonium (CH₃)₄N⁺.^[19] Gold(II) compounds are usually diamagnetic with Au-Au bonds such as [Au(CH₂)₂P(C₆H₅)₂]₂Cl₂. A noteworthy, legitimate Au(II) complex contains xenon as a ligand, [AuXe₄](Sb₂F₁₁)₂.^[20] Gold pentafluoride is the sole example of Au(V), the highest verified oxidation state.^[21]

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 that van der Waals bonding. The interaction is estimated to be comparable in strength to that of a hydrogen bond.

Mixed valence compounds

Well-defined cluster compounds are numerous.^[19] In such cases, gold has a fractional oxidation state. A representative example is the octahedral species {Au(P(C₆H₅)₃)}₆²⁺. Gold chalcogenides, e.g. "AuS" feature equal amounts of Au(I) and Au(III).

Isotopes

Main article: isotopes of gold

There is one stable isotope of gold, and 18 radioisotopes with ¹⁹⁵Au being the most stable with a half-life of 186 days.

Gold has been proposed as a "salting" material for nuclear weapons (cobalt is another, better-known salting material). A jacket of natural gold, irradiated by the intense high-energy neutron flux from an exploding thermonuclear weapon, would transmute into the radioactive isotope Au-198 with a half-life of 2.697 days and produce approximately .411 MeV of gamma radiation, significantly increasing the radioactivity of the weapon's fallout for several days. Such a weapon is not known to have ever been built, tested, or used.

Three Gold Sovereigns with a Krugerrand

Swiss-cast 1 kg gold bar.