Less common oxidation states of gold include −1, +2, and +5.
The −1 oxidation state occurs in compounds containing the Au− anion, called aurides. Caesium auride (CsAu), for example, crystallizes in the caesium chloride motif.[35] Other aurides include those of Rb+, K+, and tetramethylammonium (CH3)4N+.[36] Gold has the highest Pauling electronegativity of any metal, with a value of 2.54, making the auride anion relatively stable.
Gold(II) compounds are usually diamagnetic with Au–Au bonds such as [Au(CH2)2P(C6H5)2]2Cl2. The evaporation of a solution of Au(OH)
3 in concentrated H
2SO
4 produces red crystals of gold(II) sulfate, Au2(SO4)2. Originally thought to be a mixed-valence compound, it has been shown to contain Au4+
2 cations, analogous to the better-known mercury(I) ion, Hg2+
2 .[37][38] A gold(II) complex, the tetraxenonogold(II) cation, which contains xenon as a ligand, occurs in [AuXe4](Sb2F11)2.[39]
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.[40]
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.
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