Ammonium hexachloroiridate(IV)
[CAS# 16940-92-4]

Identification

• Classification: Inorganic chemical industry >> Inorganic salt >> Inorganic ammonium salt
• Name: Ammonium hexachloroiridate(IV)
• Synonyms: Diammonium hexachloroiridate; Iridium(IV)-ammonium chloride
• Molecular Formula: (NH₄)₂.IrCl₆
• Molecular Weight: 441.01
• CAS Registry Number: 16940-92-4
• EC Number: 241-007-0
• SMILES: [NH4+].[NH4+].Cl[Ir-2](Cl)(Cl)(Cl)(Cl)Cl

Properties

• Density: 2.86 g/mL (Expl.)

Safety Data

• Hazard Symbols: GHS05;GHS07;GHS09 Danger
• Risk Statements: H290-H302-H312-H317-H318-H332-H411
• Safety Statements: P234-P261-P264-P264+P265-P270-P271-P272-P273-P280-P301+P317-P302+P352-P304+P340-P305+P354+P338-P317-P321-P330-P333+P317-P362+P364-P390-P391-P406-P501

Hazard Classification

Hazard	Class	Category Code	Hazard Statement
Skin sensitization	Skin Sens.	1A	H317
Acute toxicity	Acute Tox.	4	H302
Acute toxicity	Acute Tox.	4	H312
Acute toxicity	Acute Tox.	4	H332
Substances or mixtures corrosive to metals	Met. Corr.	1	H290
Serious eye damage	Eye Dam.	1	H318
Eye irritation	Eye Irrit.	2	H319
Chronic hazardous to the aquatic environment	Aquatic Chronic	2	H411

Discovery and Applications

Ammonium hexachloroiridate(IV), with the chemical formula (NH₄)₂[IrCl₆], is an important coordination compound in inorganic and organometallic chemistry. It consists of the hexachloroiridate(IV) anion, [IrCl₆]²⁻, where the iridium center is in the +4 oxidation state and is octahedrally coordinated by six chloride ligands. Two ammonium cations (NH₄⁺) balance the charge of the complex anion, resulting in a crystalline salt that typically forms orange to reddish-orange solids with well-defined crystalline morphology.

The preparation of ammonium hexachloroiridate(IV) commonly starts from elemental iridium or iridium(III) compounds. A typical method involves oxidizing iridium metal or iridium(III) salts in aqueous hydrochloric acid with a strong oxidizing agent, such as chlorine gas or hydrogen peroxide, to form the iridium(IV) hexachloride complex in solution. Upon addition of ammonium chloride or ammonium salts, the hexachloroiridate(IV) complex precipitates as the ammonium salt due to its relatively low solubility compared to other species in solution. The solid is isolated by filtration, washed, and dried to yield pure ammonium hexachloroiridate(IV).

Structurally, the [IrCl₆]²⁻ anion adopts a nearly perfect octahedral geometry. The iridium(IV) center is a low-spin d⁵ ion, which confers specific electronic properties including diamagnetism. The chloride ligands symmetrically coordinate the metal, stabilizing the high oxidation state through strong metal–ligand bonds. The ammonium ions reside in the lattice, forming ionic and hydrogen bonding interactions that stabilize the crystalline solid and influence its solubility and thermal behavior.

Ammonium hexachloroiridate(IV) has significant importance as a precursor in iridium chemistry. It serves as a key starting material for the synthesis of a wide range of iridium complexes, many of which are employed as catalysts in homogeneous and heterogeneous catalysis. These catalysts facilitate important chemical transformations including hydrogenation, C–H bond activation, oxidation, and carbon–carbon coupling reactions. The ability to replace chloride ligands in [IrCl₆]²⁻ with other ligands such as phosphines, amines, or carbenes allows chemists to fine-tune the catalytic properties of the resulting complexes to suit particular synthetic challenges.

In addition to its catalytic utility, ammonium hexachloroiridate(IV) is utilized in materials science for the preparation of iridium-containing functional materials. It acts as a precursor in the fabrication of iridium thin films and nanoparticles by chemical vapor deposition or solution-phase processes. These materials are employed in electronics, sensors, energy conversion devices such as fuel cells, and as catalysts for oxygen evolution reactions. The compound’s thermal stability and well-defined coordination environment enable controlled synthesis of iridium-based materials with desired physical and chemical characteristics.

The compound is also used in analytical chemistry, especially in qualitative and quantitative methods to detect or separate iridium species. Its distinctive color and crystallinity facilitate identification and purification steps in iridium refining processes, which are critical for recovering this precious metal from ores and recycled materials.

Safety considerations are essential when handling ammonium hexachloroiridate(IV). It should be treated as a hazardous material due to its potential toxicity and irritant properties. Exposure via inhalation, ingestion, or skin contact should be avoided by using appropriate protective equipment including gloves, lab coats, and eye protection. The compound should be stored in a cool, dry place, away from incompatible chemicals such as strong reducing agents, which may cause decomposition or release of toxic gases.

Ammonium hexachloroiridate(IV) is generally stable under normal laboratory conditions but may decompose at elevated temperatures, releasing chlorine-containing species and iridium oxides. Careful thermal management is therefore necessary when using it in material fabrication or catalyst preparation.

In summary, ammonium hexachloroiridate(IV) is a crystalline salt composed of the octahedral hexachloroiridate(IV) anion paired with ammonium cations. It is a versatile and widely used precursor in iridium chemistry, playing a critical role in the synthesis of catalytic complexes and advanced materials. Its stability, well-defined structure, and reactivity make it indispensable in research and industrial applications related to iridium metal and its compounds.

References

1987. Platinum group metal sensitivity: reactivity to platinum group metal salts in platinum halide salt-sensitive workers. Annals of allergy, 59(6).
URL: https://pubmed.ncbi.nlm.nih.gov/3122607