section_path: Chemistry/Rare oxidation states
section_name: Rare oxidation states
paragraph_text: Rare oxidation states
  
  Less common oxidation states of gold include −1, +2, and +5.
  
  The −1 oxidation state occurs in aurides, compounds containing the Au anion. Caesium auride (CsAu), for example, crystallizes in the caesium chloride motif; rubidium, potassium, and tetramethylammonium aurides are also known. Gold has the highest electron affinity of any metal, at 222.8 kJ/mol, making Au a stable species, analogous to the halides.
  
  Gold also has a –1 oxidation state in covalent complexes with the group 4 transition metals, such as in titanium tetraauride and the analogous zirconium and hafnium compounds. These chemicals are expected to form gold-bridged dimers in a manner similar to titanium(IV) hydride.
  
  Gold(II) compounds are usually diamagnetic with Au–Au bonds such as Cl. The evaporation of a solution of Au(OH) in concentrated H SO produces red crystals of gold(II) sulfate, Au (SO). Originally thought to be a mixed-valence compound, it has been shown to contain Au 4+ 2 cations, analogous to the better-known mercury(I) ion, Hg 2+ 2. A gold(II) complex, the tetraxenonogold(II) cation, which contains xenon as a ligand, occurs in (Sb F). In September 2023, a novel type of metal-halide perovskite material consisting of Au and Au cations in its crystal structure has been found. It has been shown to be unexpectedly stable at normal conditions.
  
  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.
  
  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.
  
  Well-defined cluster compounds are numerous. In some cases, gold has a fractional oxidation state. A representative example is the octahedral species {Au(P(C H))} 2+ 6.