Iridium(III) chloride

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.

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 for the preparation of other iridium complexes such as Vaska's complex, trans-[IrCl(CO)(PPh3)2]. 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.

Alkene complexes such as cyclooctadiene iridium chloride dimer 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

Safety

Iridium(III) chloride is not listed under Annex I of Directive 67/548/EEC, but is listed in the inventory of the Toxic Substances Control Act (TSCA). It is also known as a mild skin and eye-irritating agent.

References

References

1. (2014). "CRC Handbook of Chemistry and Physics". CRC Press.
2. "C&L Inventory".
3. (2018). "Ullmann's Encyclopedia of Industrial Chemistry".
4. {{Greenwood&Earnshaw
5. (1965). "Zur Struktur des Iridium(III)- und des Ruthenium(III)-chlorids". [[The Science of Nature.
6. (1965). "Die Kristallstruktur von β-Iridium(III)-Chlorid". [[Zeitschrift für anorganische und allgemeine Chemie.
7. 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.
8. C. E. Housecroft and A. G. Sharpe ''Inorganic Chemistry'', p. 849.
9. Winkhaus, G.; & Singer, H. (1966). Iridium(I)-Olefinkomplexe. ''Chem. Ber.'' 99:3610–18.
10. 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. {{doi. 10.1002/9780470132463.ch5.
11. (1968). "Thermal decomposition of rhodium, iridium, and ruthenium chlorides". Journal of Catalysis.
12. (1979). "Determination of noble metals by carbon furnace atomic-absorption spectrometry. Part 1. Atom formation processes". Analyst.
13. (2003). "Substrate temperature dependent structural, optical and electrical properties of spray deposited iridium oxide thin films". Applied Surface Science.
14. Mager Stellman, J.. (1998). "Encyclopaedia of Occupational Health and Safety". International Labour Organization.