Gold Mining, Congo

Some free halogens react with gold. Gold also reacts in alkaline solutions of potassium cyanide. With mercury, it forms an amalgam.
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+.