Caesium auride

Caesium auride is the inorganic compound with the formula CsAu. It is the Cs+ salt of the unusual Au− anion.

Preparation and reactions
CsAu is obtained by heating a stoichiometric mixture of caesium and gold. The two metallic-yellow liquids react to give a transparent yellow product. Despite being a compound of two metals, CsAu lacks metallic properties since it is a salt with localized charges; it instead behaves as a semiconductor with band gap 2.6 eV.

The compound hydrolyzes readily, yielding caesium hydroxide, metallic gold, and hydrogen.


 * 2 CsAu + 2 H2O → 2 CsOH + 2 Au + H2

The solution in liquid ammonia is brown, and the ammonia adduct CsAu*NH3 is blue; the latter has ammonia molecules intercalated between layers of the CsAu crystal parallel to the (110) plane. Solutions undergo metathesis with tetramethylammonium loaded ion exchange resin to give tetramethylammonium auride.