Rhodium(III) nitrate
[CAS# 10139-58-9]

Identification

• Classification: Inorganic chemical industry >> Inorganic salt >> Metal nitrates and nitrites
• Name: Rhodium(III) nitrate
• Synonyms: rhodium(+2) cation trinitrate
• Molecular Formula: Rh.(NO₃)₃
• Molecular Weight: 288.92
• CAS Registry Number: 10139-58-9
• EC Number: 233-397-6
• SMILES: [N+](=O)([O-])[O-].[N+](=O)([O-])[O-].[N+](=O)([O-])[O-].[Rh+3]

Safety Data

• Hazard Symbols: GHS03;GHS05;GHS07;GHS08;GHS09 Danger
• Risk Statements: H271-H272-H290-H302-H314-H317-H318-H341-H400-H410
• Safety Statements: P203-P210-P220-P234-P260-P261-P264-P264+P265-P270-P272-P273-P280-P283-P301+P317-P301+P330+P331-P302+P352-P302+P361+P354-P304+P340-P305+P354+P338-P306+P360-P316-P317-P318-P321-P330-P333+P317-P362+P364-P363-P370+P378-P371+P380+P375-P390-P391-P405-P406-P420-P501

Hazard Classification

Hazard	Class	Category Code	Hazard Statement
Skin corrosion	Skin Corr.	1B	H314
Substances or mixtures corrosive to metals	Met. Corr.	1	H290
Chronic hazardous to the aquatic environment	Aquatic Chronic	1	H410
Acute toxicity	Acute Tox.	4	H302
Serious eye damage	Eye Dam.	1	H318
Acute hazardous to the aquatic environment	Aquatic Acute	1	H400
Germ cell mutagenicity	Muta.	2	H341
Skin sensitization	Skin Sens.	1A	H317
Oxidising solids	Ox. Sol.	1	H271
Oxidising solids	Ox. Sol.	3	H272
Skin sensitization	Skin Sens.	1	H317
Skin corrosion	Skin Corr.	1A	H314
Oxidising solids	Ox. Sol.	2	H272
Acute toxicity	Acute Tox.	4	H332

• Transport Information: UN 3085

Discovery and Applications

Rhodium(III) nitrate is an inorganic compound composed of rhodium in the +3 oxidation state and nitrate anions, typically represented by the empirical formula Rh(NO₃)₃. It is usually encountered as a hydrate or in solution, rather than as a pure anhydrous solid, and serves as a source of rhodium(III) ions for applications in catalysis, materials chemistry, and coordination compound synthesis.

The compound is typically prepared by dissolving rhodium metal, rhodium(III) oxide (Rh₂O₃), or rhodium(III) hydroxide in concentrated nitric acid. This reaction results in the formation of a red or reddish-brown solution containing rhodium nitrate species. The exact stoichiometry of the nitrate complex can vary depending on the preparation method and concentration of nitric acid, and the product is often isolated as a hydrate or used in situ for further chemical transformations.

In aqueous solution, rhodium(III) nitrate does not usually exist as a discrete molecular salt but as a mixture of solvated rhodium complexes coordinated by nitrate and water ligands. Rhodium(III) typically forms six-coordinate, octahedral complexes. In concentrated nitric acid, nitrato ligands (NO₃⁻) may coordinate to rhodium directly, while in more dilute conditions, water ligands dominate the coordination sphere. The solution chemistry of rhodium(III) nitrate is complex and involves equilibria among multiple species.

One of the main applications of rhodium(III) nitrate is as a precursor for the synthesis of rhodium-based catalysts. These catalysts are widely used in homogeneous and heterogeneous catalysis for hydrogenation, hydroformylation, carbon-carbon bond formation, and oxidation reactions. Rhodium nitrate solutions are used to impregnate supports such as alumina, silica, or activated carbon, followed by calcination or reduction to yield catalytically active rhodium metal or oxide species.

Rhodium(III) nitrate is also used in the preparation of rhodium coordination complexes, particularly those involving nitrogen- and phosphorus-donor ligands. These complexes are of interest for their catalytic, electronic, and optical properties. In analytical chemistry, rhodium(III) nitrate can be used as a reagent in spectroscopic methods, such as atomic absorption spectroscopy, where rhodium is employed as a matrix modifier.

In materials science, rhodium(III) nitrate may serve as a precursor in thin-film deposition, ceramics, and rhodium-containing conductive coatings. When thermally decomposed, it forms rhodium(III) oxide or metallic rhodium, depending on the atmosphere and temperature, making it suitable for fabrication of materials with catalytic or electronic functionality.

Thermal decomposition of rhodium(III) nitrate involves the stepwise loss of water (if hydrated), followed by decomposition of nitrate ligands, which releases nitrogen oxides (NO, NO₂) and results in the formation of rhodium oxide or elemental rhodium. These properties are used in controlled heating protocols to produce rhodium-containing films and powders.

Rhodium(III) nitrate must be handled with care due to the toxicity of rhodium salts and the oxidizing nature of nitrate. Direct contact and inhalation of dust or vapors should be avoided. Solutions should be stored in well-sealed containers away from light, heat, and incompatible materials such as reducing agents or organics. Proper disposal procedures must be followed for rhodium-containing waste due to its environmental persistence and high economic value.

In summary, rhodium(III) nitrate is a valuable rhodium(III) compound primarily used as a precursor in the synthesis of catalysts and coordination complexes. Its solubility in nitric acid, ability to form diverse solution species, and reactivity with ligands make it useful in various fields including synthetic chemistry, catalysis, and materials science.

References

2021. High-throughput computational-experimental screening protocol for the discovery of bimetallic catalysts. npj Computational Materials, 7(1).
DOI: 10.1038/s41524-021-00605-6

2019. Active TiO2-Nanostructured Surfaces for CO Oxidation on Rh Model Catalysts at Low-Temperature. Catalysis Letters, 149(6).
DOI: 10.1007/s10562-019-02757-z

2014. CO2/H2 methanation over M*/Mn/Fe-Al2O3 (M*: Pd, Rh, and Ru) catalysts in natural gas; optimization by response surface methodology-central composite design. Clean Technologies and Environmental Policy, 16(7).
DOI: 10.1007/s10098-014-0814-8