Ammonium hexachlororhodate(III)
[CAS# 15336-18-2]

Identification

• Classification: Inorganic chemical industry >> Inorganic salt >> Inorganic ammonium salt
• Name: Ammonium hexachlororhodate(III)
• Synonyms: Triammonium hexachlororhodate
• Molecular Formula: (NH₄)₃.RhCl₆
• Molecular Weight: 369.74
• CAS Registry Number: 15336-18-2
• EC Number: 239-364-2
• SMILES: [NH4+].[NH4+].[NH4+].Cl[Rh-3](Cl)(Cl)(Cl)(Cl)Cl

Properties

• Water solubility: soluble

Safety Data

• Hazard Symbols: GHS05;GHS07 Danger
• Risk Statements: H315-H318-H319
• Safety Statements: P264-P264+P265-P280-P302+P352-P305+P351+P338-P305+P354+P338-P317-P321-P332+P317-P337+P317-P362+P364

Hazard Classification

Hazard	Class	Category Code	Hazard Statement
Serious eye damage	Eye Dam.	1	H318
Skin irritation	Skin Irrit.	2	H315
Eye irritation	Eye Irrit.	2	H319

Discovery and Applications

Ammonium hexachlororhodate(III) is an inorganic coordination compound with the formula (NH₄)₃[RhCl₆]. It consists of an octahedral complex anion [RhCl₆]³⁻, where a central rhodium ion in the +3 oxidation state is coordinated by six chloride ligands, balanced by three ammonium cations (NH₄⁺) to maintain charge neutrality. The compound typically appears as dark red or purple crystalline solids and is sparingly soluble in water but insoluble in most organic solvents.

The synthesis of ammonium hexachlororhodate(III) generally involves treating rhodium metal or rhodium(III) oxide with molten ammonium chloride and chlorine gas or dissolving rhodium(III) chloride in ammonium chloride solution under controlled conditions. The reaction results in the formation of the stable [RhCl₆]³⁻ anion complex, which can be isolated by crystallization. The compound’s structure features a central low-spin d⁶ rhodium(III) ion surrounded symmetrically by six chloride ions in an octahedral geometry, leading to high stability due to ligand field stabilization.

Ammonium hexachlororhodate(III) serves as a key precursor in the preparation of various rhodium complexes and catalysts. Due to its stability and well-defined coordination environment, it is often used as a starting material in homogeneous catalysis research. For example, it can be converted into rhodium(I) and rhodium(III) complexes bearing phosphine, carbonyl, or nitrogen donor ligands by ligand substitution reactions. These derived complexes are widely employed as catalysts in hydrogenation, hydroformylation, and carbon–carbon coupling reactions, which are fundamental in synthetic organic chemistry and industrial processes.

The compound also plays a role in materials science and analytical chemistry. It can be used as a rhodium source in electrodeposition processes to prepare rhodium metal coatings with excellent corrosion resistance and catalytic properties. In analytical applications, ammonium hexachlororhodate(III) is utilized in qualitative and quantitative analysis for the detection and determination of rhodium and related metals.

Handling of ammonium hexachlororhodate(III) requires caution due to its content of heavy metals and chloride ions. It is stable under normal conditions but should be stored in a dry environment away from reducing agents and strong bases to prevent decomposition. Exposure to dust or solutions should be minimized with appropriate protective equipment, as rhodium compounds may pose health risks upon inhalation or ingestion.

In summary, ammonium hexachlororhodate(III) is a dark crystalline salt composed of the octahedral [RhCl₆]³⁻ anion balanced by ammonium cations. It is an important, stable rhodium(III) complex used as a precursor for synthesizing catalytic rhodium complexes, electrodeposition materials, and in analytical chemistry.

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