Iridium(III) chloride is the inorganic compound with the formula IrCl3. The anhydrous compound is relatively rare. It has two polymorphs, α and β, which are brown and red colored respectively. The related trihydrate, IrCl3(H2O)3, is hygroscopic, dark green, and much more commonly encountered, being a starting point for iridium chemistry.
Preparation 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.
Hydrated iridium trichloride is obtained by heating hydrated iridium(III) oxide with hydrochloric acid.
Structure Like the related rhodium compound, IrCl3 adopts the structure seen for aluminium chloride. This is the monoclinic α polymorph. A rhombohedral β polymorph also exists. Both polymorphs have effectively the same anion lattice but differ in the octahedral interstices the iridium ions occupy. The α polymorph converts to the β polymorph when heated to around 650 °C.
+ Crystallographic data for the 2 polymorphs of iridium(III) chlorideCompoundα-IrCl3β-IrCl3Crystal StructureMonoclinicOrthorhombicSpace GroupC2/mFdddLattice constant a (Å)5.996.95Lattice constant b (Å)10.379.81Lattice constant c (Å)5.9920.82β109.4°Calculated density (g/cm3)5.335.34
The structure of the trihydrate has not been elucidated yet.
Reactions and uses 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].Vaska, L.; & DiLuzio, J. W. (1961) J. Am. Chem. Soc. 83:2784. Girolami, G.S.; Rauchfuss, T.B.; Angelici, R.J. (1999). Synthesis and Technique in Inorganic Chemistry (3rd Edn.). Sausalito: University Science Books. 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.C. E. Housecroft and A. G. Sharpe Inorganic Chemistry, p. 849.
Alkene complexes such as cyclooctadiene iridium chloride dimerWinkhaus, G.; & Singer, H. (1966). Iridium(I)-Olefinkomplexe. Chem. Ber. 99:3610–18.Herde, J. L.; Lambert, J. C.; & Senoff, C. V. (1974). Cyclooctene and 1,5-Cyclooctadiene Complexes of Iridium(I). Inorg. Synth. 1974, volume 15, pages 18–20. . and chlorobis(cyclooctene)iridium dimer can also be prepared by heating the hydrate with the appropriate alkene in water/alcohol mixtures.
Decomposition The trihydrate decomposes to the anhydrous form at 200 °C, which then oxidizes in air at 763 °C to iridium(IV) oxide, which then decomposes to iridium metal at 1070 °C. However, under hydrogen, it is directly reduced at 190 °C to iridium metal: 2 IrCl3 + 3 H2 → 2 Ir + 6 HCl
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