Gold Mining, Congo

Less common oxidation states
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. Other aurides include those of Rb+, K+, and tetramethylammonium (CH3)4N+.
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 H2SO4 produces red crystals of gold(II) sulfate, AuSO4. Originally thought to be a mixed-valence compound, it has been shown to contain Au4+