Hexachloroiridic acid
[CAS# 16941-92-7]

Identification

• Classification: Inorganic chemical industry >> Inorganic acid
• Name: Hexachloroiridic acid
• Synonyms: Hydrogen hexachloroiridate (IV)
• Molecular Formula: H₂IrCl₆
• Molecular Weight: 406.95
• CAS Registry Number: 16941-92-7
• EC Number: 241-012-8
• SMILES: [H+].[H+].Cl[Ir-2](Cl)(Cl)(Cl)(Cl)Cl

Safety Data

• Hazard Symbols: GHS05;GHS07;GHS09 DangerGHS05
• Hazard Statements: H290-H302-H314-H315-H317-H318-H319-H335-H411
• Precautionary Statements: P234-P260-P261-P264-P264+P265-P270-P271-P272-P273-P280-P301+P317-P301+P330+P331-P302+P352-P302+P361+P354-P304+P340-P305+P351+P338-P305+P354+P338-P316-P317-P319-P321-P330-P332+P317-P333+P317-P337+P317-P362+P364-P363-P390-P391-P403+P233-P405-P406-P501

Hazard Classification

Hazard	Class	Category Code	Hazard Statement
Eye irritation	Eye Irrit.	2	H319
Skin irritation	Skin Irrit.	2	H315
Acute toxicity	Acute Tox.	4	H302
Skin sensitization	Skin Sens.	1	H317
Chronic hazardous to the aquatic environment	Aquatic Chronic	2	H411
Specific target organ toxicity - single exposure	STOT SE	3	H335
Substances or mixtures corrosive to metals	Met. Corr.	1	H290
Skin corrosion	Skin Corr.	1C	H314
Serious eye damage	Eye Dam.	1	H318
Skin corrosion	Skin Corr.	1B	H314
Acute toxicity	Acute Tox.	4	H332
Acute toxicity	Acute Tox.	4	H312
Carcinogenicity	Carc.	2	H351

• Transport Information: UN 3260

Discovory and Applicatios

Hexachloroiridic acid, chemically represented as H₂IrCl₆, is an inorganic coordination compound widely used as a key precursor in iridium chemistry. It consists of the hexachloroiridate(IV) anion, \[IrCl₆]²⁻, where iridium is in the +4 oxidation state and is octahedrally coordinated by six chloride ligands. The two protons (H⁺) balance the charge of the complex, making it an acidic compound. Hexachloroiridic acid typically appears as a reddish-orange crystalline solid or as an aqueous solution exhibiting acidic properties.

The preparation of hexachloroiridic acid generally involves the dissolution of elemental iridium or iridium-containing alloys in aqua regia, a mixture of concentrated nitric acid and hydrochloric acid. This aggressive oxidizing mixture converts iridium metal into its +4 oxidation state and coordinates it with chloride ions, yielding the hexachloroiridate(IV) complex ion in solution. The aqueous solution containing this complex is then isolated, and through controlled evaporation or crystallization, pure hexachloroiridic acid crystals can be obtained.

Structurally, the hexachloroiridate(IV) anion is octahedral, featuring an iridium atom surrounded symmetrically by six chloride ions. The strong metal–ligand bonds stabilize the iridium center in the +4 oxidation state. The presence of two acidic protons makes the compound behave as a strong acid in aqueous solutions, allowing it to donate protons readily. This acidic nature distinguishes hexachloroiridic acid from its ammonium or potassium salt counterparts, which contain the same complex anion but with different charge-balancing cations.

Hexachloroiridic acid serves as a versatile precursor for synthesizing a wide array of iridium complexes. Its solubility and well-defined coordination environment facilitate ligand substitution reactions, enabling the replacement of chloride ligands with a variety of donor molecules, such as phosphines, amines, or N-heterocyclic carbenes. This flexibility is fundamental in preparing tailored iridium catalysts used in homogeneous catalysis. Such catalysts find applications in industrial processes including hydrogenation, C–H bond activation, asymmetric synthesis, and oxidation reactions.

In the field of materials science, hexachloroiridic acid is used as a source of iridium in the fabrication of iridium-containing thin films, coatings, and nanoparticles. Techniques such as chemical vapor deposition (CVD) and solution-based synthesis rely on the compound as a metal precursor due to its volatility and chemical stability. The resulting iridium-based materials are employed in electronics, fuel cells, and as catalysts for oxygen evolution and other electrochemical reactions. The ability to precisely control the deposition and composition of iridium layers makes hexachloroiridic acid critical for advanced materials engineering.

From an analytical perspective, hexachloroiridic acid plays a role in determining iridium content in various samples. Its distinctive color and reactivity allow it to be used in titrations, colorimetric assays, and purification protocols. The compound's formation during the digestion of platinum-group metals is also an indicator of iridium presence and helps guide extraction and refining processes.

Safety and handling considerations are important when working with hexachloroiridic acid. The compound is strongly acidic and corrosive, posing risks of chemical burns and inhalation hazards. Appropriate protective equipment such as gloves, goggles, and lab coats must be used, along with working in well-ventilated areas or fume hoods. Storage requires sealed containers in cool, dry environments, away from reducing agents and incompatible chemicals to prevent hazardous reactions.

The thermal stability of hexachloroiridic acid is moderate; it can decompose upon heating, releasing chlorine-containing gases and leaving iridium oxides or metallic iridium residues. Therefore, thermal processes involving the compound should be carefully controlled to avoid safety risks and ensure desired product formation.

In summary, hexachloroiridic acid is a reddish-orange inorganic acid composed of the octahedral hexachloroiridate(IV) complex anion and two protons. It serves as a fundamental precursor for the synthesis of iridium coordination complexes, catalysts, and advanced materials. Its well-defined structure, acidity, and versatility underpin its extensive use in both academic research and industrial applications involving iridium chemistry.

References

2020. Oxidations of Benzhydrazide and Phenylacetic Hydrazide by Hexachloroiridate(IV): Reaction Mechanism and Structure�Reactivity Relationship. Molecules (Basel, Switzerland), 25(2).
DOI: 10.3390/molecules25020308

2014. Oxidation of Cysteinesulfinic Acid by Hexachloroiridate(IV). The Journal of Physical Chemistry B, 118(2).
DOI: 10.1021/jp4116723

2012. Oxidation of Glutathione by Hexachloroiridate(IV), Dicyanobis(bipyridine)iron(III), and Tetracyano(bipyridine)iron(III). Inorganic Chemistry, 51(22).
DOI: 10.1021/ic301955y