AuCl3 exists as a chloride-bridged both as a solid and as a vapour, at least at low temperatures. Gold bromide behaves analogously. The structure is similar to that of iodine chloride. In gold chloride, each gold center is square planar, which is typical of a metal complex with a d8 electron count. The bonding in AuCl3 is considered somewhat covalent.
Preparation
Gold chloride is most often prepared by passingchlorine gas over gold powder at 180 °C: Another method of preparation is by reacting Au3+ species with chloride to produce tetrachloroaurate. Its acid, chloroauric acid, is then heated to eliminate hydrogen chloride gas. Reaction with aqua regia produces gold chloride:
Reactions
On contact with water, forms acidic hydrates and the conjugate base. It may be reduced by iron| causing elemental gold to be precipitated from solution. Anhydrous AuCl3 begins to decompose to AuCl at around 160 °C; however, this in turn undergoes disproportionation at higher temperatures to give gold metal and AuCl3. AuCl3 is Lewis acidic and readily forms complexes. For example, it reacts with hydrochloric acid to form chloroauric acid : Other chloride sources, such as KCl, also convert AuCl3 into. Aqueous solutions of AuCl3 react with aqueous base such as sodium hydroxide to form a precipitate of Au3, which will dissolve in excess NaOH to form sodium aurate. If gently heated, Au3 decomposes to gold oxide, Au2O3, and then to gold metal. Gold chloride is the starting point for the synthesis of many other gold compounds. For example, reaction with potassium cyanide produces the water-soluble complex, K:
AuCl3 has attracted the interest of organic chemists as a mild acid catalyst for a variety of reactions, although no transformations have been commercialized. Gold salts, especially Na, provide an alternative to mercury salts as catalysts for reactions involving alkynes. An illustrative reaction is the hydration of terminal alkynes to produce acetyl compounds. Some alkynes undergo amination in the presence of gold catalysts. Gold catalyses the alkylation of certain aromatic rings and a conversion of furans to phenols. For example, a mixture of acetonitrile and gold chloride catalyses the alkylation of 2-methylfuran by methyl vinyl ketone at the 5-position: The efficiency of this organogold reaction is noteworthy because both the furan and the ketone are sensitive to side-reactions such as polymerisation under acidic conditions. In some cases where alkynes are present, phenols sometimes form : This reaction involves a rearrangement that gives a new aromatic ring. As a stoichiometric reagent, auric chloride reacts with benzene under extremely mild conditions to afford the dimeric phenylgold dichloride:
