The neutral monomer , as well as the other neutral gold trihalide species, has not been isolated in the gas phase which indicates the coordination number three is not favored. Predominantly, gold(III) displays square planar coordination corresponding to a preferred coordination number of four.
Specifically, in solution gold(III) trihalides have the tendency to add a fourth ligand to form the more preferred four-coordinate complex. With respect to gold tribromide, it is common to purchase gold(III) bromide hydrate, , where the central gold atom exhibits a coordination number of four, rather than the anhydrous form of the compound, which exhibits a coordination number of three.
Alternatively, if there is no addition of a fourth ligand, gold tribromide will oligomerize to form the halogen-bridged dimer complex mentioned previously.
Furthermore, like gold(III) chloride, gold tribromide is a Lewis acid and can form several complexes. For example, in the presence of hydrobromic acid, the dimer dissolves and bromoauric acid is formed.
The dimer also undergoes hydrolysis rapidly in moist air.