Iridium(III) chloride

Iridium(III) chloride is the inorganic compound with the formula IrCl3. The anhydrous compound is relatively rare, but the related hydrate is much more commonly encountered. The anhydrous salt has two polymorphs, α and β, which are brown and red colored respectively. More commonly encountered is the hygroscopic dark green trihydrate IrCl3(H2O)3 which is a common starting point for iridium chemistry.[4]

Iridium(III) chloride

α-IrCl3

β-IrCl3
Iridium(III) chloride hydrate
Iridium(III) chloride trihydrate
Names
Other names
Iridium trichloride
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.030.028
EC Number
  • 233-044-6
UNII
  • InChI=1S/3ClH.Ir/h3*1H;/q;;;+3/p-3
    Key: DANYXEHCMQHDNX-UHFFFAOYSA-K
  • [Cl-].[Cl-].[Cl-].[Ir+3]
Properties
IrCl3
Molar mass 298.58 g/mol (anhydrous)
Appearance brown solid (α-anhydrous)
red solid (β-anhydrous)
dark green solid (trihydrate)
Density 5.30 g/cm3, solid[1]
Melting point 763 °C (1,405 °F; 1,036 K)[1][2] (decomposes)
insoluble (anhydrous IrCl3), soluble (hydrated derivative)[1]
Solubility Insoluble in HCl and alkanes[1]
−14.4·10−6 cm3/mol
Structure
Monoclinic, mS16
C12/m1, No. 12
Thermochemistry
-257 kJ/mol
Hazards
GHS labelling:[3]
GHS07: Exclamation markGHS09: Environmental hazard
Warning
H302, H411
Flash point non-flammable
Related compounds
Other cations
 Rhodium(III) chloride
Related compounds
 Platinum(II) chloride
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

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.[5]

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

Structure

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]

Crystallographic data for the 2 polymorphs of iridium(III) chloride
Compound α-IrCl3[7] β-IrCl3[8]
Crystal Structure Monoclinic Orthorhombic
Space Group C2/m Fddd
Lattice constant a (Å) 5.99 6.95
Lattice constant b (Å) 10.37 9.81
Lattice constant c (Å) 5.99 20.82
β 109.4°
Calculated density (g/cm3) 5.33 5.34

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].[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.

Decomposition

The trihydrate decomposes to the anhydous 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][12][13]

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.[14]

References
  1. Haynes, William, ed. (2014). CRC Handbook of Chemistry and Physics. CRC Press. p. 4-68. ISBN 9781482208689.
  2. A.E. Newkirk; D.W. McKee (1968). "Thermal decomposition of rhodium, iridium, and ruthenium chlorides". Journal of Catalysis. 11 (4): 370–377. doi:10.1016/0021-9517(68)90061-4.
  3. "C&L Inventory". echa.europa.eu. Retrieved 23 December 2021.
  4. C. E. Housecroft and A. G. Sharpe Inorganic Chemistry, p. 849.
  5. Hermann Renner (2018). "Platinum Group Metals and Compounds". Ullmann's Encyclopedia of Industrial Chemistry: 1–73. doi:10.1002/9783527306732.a21_075.pub2. ISBN 9783527303854. S2CID 94472506.
  6. Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
  7. Brodersen, K.; Moers, F.; Schnering, H. G. (1965). "Zur Struktur des Iridium(III)- und des Ruthenium(III)-chlorids". Naturwissenschaften. 52 (9): 205–206. doi:10.1007/BF00626459. S2CID 43351743.
  8. Meisel, A.; Leonhardt, G. (1965). "Die Kristallstruktur von β-Iridium(III)-Chlorid". Z. anorg. allg. Chem. 339 (1–2): 57–66. doi:10.1002/zaac.19653390109.
  9. 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.
  10. Winkhaus, G.; & Singer, H. (1966). Iridium(I)-Olefinkomplexe. Chem. Ber. 99:3610–18.
  11. 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.
  12. W. B. Rowston; J. M. Ottaway (1979). "Determination of noble metals by carbon furnace atomic-absorption spectrometry. Part 1. Atom formation processes". Analyst. 104 (1240): 645–659. Bibcode:1979Ana...104..645R. doi:10.1039/AN9790400645.
  13. R. K. Kawar; P. S. Chigare; P. S. Patil (2003). "Substrate temperature dependent structural, optical and electrical properties of spray deposited iridium oxide thin films". Applied Surface Science. 206 (1–4): 90–101. Bibcode:2003ApSS..206...90K. doi:10.1016/S0169-4332(02)01191-1.
  14. Mager Stellman, J. (1998). "Iridium". Encyclopaedia of Occupational Health and Safety. International Labour Organization. pp. 63.19. ISBN 978-92-2-109816-4. OCLC 35279504.
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