Iridium(III) chloride

Iridium is separated from the other platinum group metals as crystalline ammonium hexachloroiridate, (NH₄)₂[IrCl₆], which can be reduced to iridium metal in a stream of hydrogen. The spongy Ir thus produced reacts with chlorine at 650 °C to give iridium(III) chloride.^[5]

Hydrated iridium trichloride is obtained by heating hydrated iridium(III) oxide with hydrochloric acid.^[6]

Like the related rhodium compound, IrCl₃ adopts the structure seen for aluminium chloride.^[6] This is the monoclinic α polymorph.^[7] A rhombohedral β polymorph also exists. Both polymorphs have effectively the same anion lattice but differ in the octahedral interstices the iridium ions occupy.^[8] The α polymorph converts to the β polymorph when heated to around 650 °C.^[4]

α-IrCl₃

β-IrCl₃

Structures of the forms of iridium(III) chloride

The structure of the trihydrate has not been elucidated yet.^{[as of?]}

Industrially, most iridium complexes are generated from ammonium hexachloroiridate or the related chloroiridic acid (H₂IrCl₆). The Cativa process, source of most of the world's acetic acid relies on such catalysts.

Hydrated iridium(III) chloride is used in the preparation of other iridium complexes such as Vaska's complex, trans-[IrCl(CO)(PPh₃)₂].^[9] With the presence of the chloride anion, it forms hexachloroiridate(III), and produces hexachloroiridate(IV) in aqua regia. The trihydrate react with ammonia to form ammine complexes, such as pentaamminechloroiridium(III) chloride, formulated [IrCl(NH₃)₅]Cl₂. It also reacts with concentrated ammonium hydroxide at 150 °C to form the fully ammoniated complex, [Ir(NH₃)₆]Cl₃. The hydrate can also form complexes upon reaction with bipyridine, acetonitrile, and pyridine.^[4]

Alkene complexes such as cyclooctadiene iridium chloride dimer^[10][11] and chlorobis(cyclooctene)iridium dimer^[11][10] can also be prepared by heating the hydrate with the appropriate alkene in water/alcohol mixtures.