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

Iridium(III) chloride trihydrate Trihydrate
Names
Other names Iridium trichloride
Identifiers
CAS Number	10025-83-9 (anhydrous) Y 14996-61-3 (hydrate)
3D model (JSmol)	Interactive image
ChemSpider	23837 Y
ECHA InfoCard	100.030.028
EC Number	233-044-6
PubChem CID	82301
UNII	20278OEG45 Y
CompTox Dashboard (EPA)	DTXSID10275787
InChI InChI=1S/3ClH.Ir/h3*1H;/q;;;+3/p-3 Key: DANYXEHCMQHDNX-UHFFFAOYSA-K
SMILES [Cl-].[Cl-].[Cl-].[Ir+3]
Properties
Chemical formula	IrCl3
Molar mass	298.58 g/mol (anhydrous) 316.60 g/mol (hydrate)
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)
Solubility in water	insoluble (anhydrous IrCl3), soluble (hydrated derivative)[1]
Solubility	Insoluble in HCl and alkanes[1]
Magnetic susceptibility (χ)	−14.4·10−6 cm3/mol
Structure
Crystal structure	Monoclinic, mS16
Space group	C12/m1, No. 12
Thermochemistry
Std enthalpy of formation (ΔfH⦵298)	−257 kJ/mol
Hazards
GHS labelling:[3]
Pictograms
Signal word	Warning
Hazard statements	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). N verify (what is YN ?) 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]

α-IrCl₃

β-IrCl₃

Structures of the forms of iridium(III) chloride

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

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].^[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 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][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]