Caesium auride

Caesium auride (CsAu) is an ionic compound containing the unusual Au ion first discovered in 1978 in the laboratory of Joseph Lagowski. It is obtained by heating a stoichiometric mixture of caesium and gold; the two metallic-yellow liquids react to give a clear product. The solution in liquid ammonia is brown, and the solid is yellow (the colour of both metals making up the compound); the ammonium adduct is dark blue. Despite being a compound of two metals, CsAu lacks metallic properties since the free electrons in both metals are used up, as in all ionic compounds.