What is the most common oxidation state of rhodium?

The +3 state is the most common and thermodynamically stable for rhodium in ordinary compounds, seen in RhCl3, Rh2O3, and Rh(acac)3. The +1 state is also very important in organometallic and catalytic chemistry.

Why is rhodium +1 important in catalysis?

Rh(I) complexes such as Wilkinson's catalyst [RhCl(PPh3)3] and the Monsanto acetic acid process catalyst are highly effective for hydrogenation and carbonylation reactions. The Rh(I)/Rh(III) redox cycle underpins oxidative addition and reductive elimination steps central to these catalytic cycles.

What is rhodium's electron configuration?

Rhodium has an anomalous ground-state configuration of [Kr] 4d8 5s1 rather than the expected [Kr] 4d7 5s2, because the slightly higher stability of having more electrons in the 4d shell outweighs the 5s2 pairing. When forming Rh3+, all three 4d/5s outer electrons are removed to give [Kr] 4d6.

Does rhodium show negative oxidation states?

Yes, a −1 state has been documented in metal carbonyl cluster anions such as [Rh(CO)4]−. These are rare and exist only in low-oxidation-state organometallic environments with strong π-acceptor ligands like CO.

How does rhodium differ from its neighbors palladium and ruthenium?

Ruthenium (Ru, Z=44) reaches higher oxidation states more readily (up to +8 in RuO4), while palladium (Pd, Z=46) strongly favors +2. Rhodium sits between them, with +3 dominant but +1 uniquely accessible for catalysis — a combination not well-matched by its neighbors.

What is rhodium used for commercially?

About 80% of rhodium production goes into three-way automotive catalytic converters, where it reduces NOx emissions. It is also used in industrial catalysis (acetic acid synthesis), jewelry alloys, hard-facing electroplating, and high-temperature furnace windings.

Rhodium Oxidation States

Name: Rhodium Oxidation States
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Author: Nham Vu

Reference for rhodium oxidation states: Rh most commonly shows +3, with +1 critical in catalysis, and rarer +2, +4, +5, +6 states documented.

Atomic # 45 Rh Rhodium

Atomic Mass

102.906 u

Group

9 (VIIIB)

Period

Block

d-block

Electronegativity

2.28 (Pauling)

Oxidation States

−1 to +6 (+3 dom.)

Rhodium is a Period 5 d-block metal with ground-state configuration [Kr] 4d⁸ 5s¹ (anomalous — one 5s electron instead of two). It exhibits oxidation states from −1 to +6, with +3 the most stable in ordinary inorganic compounds and +1 essential in homogeneous catalysis. The wide range reflects the accessibility of 4d electrons and the flexibility of d-orbital participation in bonding.

State	Stability	Example Species	Notes
+3	Stable	RhCl₃, Rh₂O₃, [Rh(NH₃)₆]³⁺	Most stable and common state. Low-spin d6 electronic configuration gives exceptional kinetic stability in octahedral complexes.
+1	Stable	[RhCl(PPh₃)₃], [Rh(CO)₂Cl]₂	Square-planar d8 geometry. Critical in organometallic catalysis — Wilkinson's catalyst and the Monsanto acetic acid process.
+2	Moderate	Rh₂(OAc)₄, RhCl₂	Dirhodium(II) paddlewheel complexes (e.g., Rh₂(OAc)₄) are stable and widely used as carbene-transfer catalysts in organic synthesis.
+4	Limited	RhO₂, RhF₄	Accessible in fluorides and high-valent oxides. Less common than +3; Rh(IV) has strong oxidising character.
+5	Rare	RhF₅	Known mainly in pentafluoride; strongly oxidising and moisture-sensitive.
+6	Very rare	RhF₆	Highest known state; a volatile, extremely reactive hexafluoride. Powerful oxidant; decomposes in water.
0	Elemental only	Rh metal	Assigned to pure rhodium by convention. Also appears in Rh(0) cluster compounds and some metal-on-support catalysts.
−1	Very rare	[Rh(CO)₄]⁻	Found in carbonyl anion clusters. Strong π-acceptor CO ligands stabilise the negative formal charge. Organometallic curiosity.

Ionization Energies
IE₁ = 719.7 kJ/mol | IE₂ = 1740 kJ/mol | IE₃ = 2997 kJ/mol
Three successive ionizations access the +3 state. The relatively manageable gap between IE₁ and IE₂ underpins the stability of both +1 and +3, compared to metals that show a single dominant state.

Rhodium forms well-characterized compounds across several oxidation states. The +3 state dominates inorganic chemistry; the +1 state dominates organometallic chemistry. Dirhodium(II) paddlewheel compounds occupy a unique niche in synthetic organic chemistry.

Compound	Formula	Rh State	Notes
Rhodium(III) Chloride	RhCl₃	+3	Most common Rh starting material. Dark red solid; used to prepare other Rh compounds and catalysts. Both anhydrous and hydrated forms exist.
Rhodium(III) Oxide	Rh₂O₃	+3	Grey solid; stable at moderate temperatures, converts to Rh metal above ~1100 °C. Used as a pigment and ceramic colorant.
Rhodium(III) Acetylacetonate	Rh(acac)₃	+3	Air-stable, soluble precursor widely used in catalysis and chemical vapor deposition of Rh thin films.
Wilkinson's Catalyst	[RhCl(PPh₃)₃]	+1	Landmark square-planar Rh(I) complex; Nobel Prize-winning catalyst for homogeneous hydrogenation of alkenes and alkynes.
Dirhodium Tetraacetate	Rh₂(OAc)₄	+2	Paddlewheel-structure dimer with Rh–Rh bond; premier catalyst for metal-carbene reactions (cyclopropanation, C–H insertion) in organic synthesis.
Rhodium Hexafluoride	RhF₆	+6	Volatile, dark-brown solid; one of the most reactive fluorides known. Reacts explosively with water. Highest oxidation state of Rh.
Rhodium Pentafluoride	RhF₅	+5	Strong fluorinating agent; forms tetrameric (RhF₅)₄ in solid state. Moisture-sensitive.
Rhodium Tetrafluoride	RhF₄	+4	Less reactive than RhF₅; one of few stable +4 Rh compounds outside of oxide phases.
Tetrakis(carbonyl)rhodate(−1)	[Rh(CO)₄]⁻	−1	Anion formed in carbonyl cluster chemistry; rare example of negative Rh state stabilised by π-backbonding.
Hexaaminorhodium(III)	[Rh(NH₃)₆]³⁺	+3	Classic octahedral Werner complex; low-spin d6 configuration makes it kinetically inert — ligand substitution is extremely slow.

Ground State Configuration

Core

Core (completes [Ar])

Core

Core (completes [Kr])

Valence — 8 of 10 filled (anomalous 5s¹ draws one here)

Valence — 1 electron (anomalous; expected 5s²)

Notation

Full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d⁸ 5s¹

Noble gas: [Kr] 4d⁸ 5s¹

Rh³⁺ ion: [Kr] 4d⁶

Rh¹⁺ ion: [Kr] 4d⁸

Anomalous configuration: Rhodium adopts [Kr] 4d⁸ 5s¹ rather than the expected [Kr] 4d⁷ 5s². Moving one 5s electron into 4d reduces electron-electron repulsion in the 5s shell enough to lower total energy. Similar anomalies occur in Pd, Cu, Cr, and Mo.

Ionization Steps to Rh³⁺

[Kr] 4d⁸ 5s¹

Neutral — 9 outer electrons (anomalous)

Rh⁺

[Kr] 4d⁸

Remove 5s¹ (IE₁ 719.7 kJ/mol); d8 sq. planar Rh(I) chemistry begins here

Rh²⁺

[Kr] 4d⁷

Remove one 4d electron (IE₂ ~1740 kJ/mol); Rh(II) in dirhodium paddlewheel compounds

Rh³⁺

[Kr] 4d⁶

Remove another 4d electron (IE₃ ~2997 kJ/mol); low-spin d6 — kinetically inert octahedral complexes

Why is Rh(III) kinetically inert?
Rh³⁺ has a low-spin d⁶ configuration in strong-field ligand environments. All t_2g orbitals are filled; ligand substitution requires passing through a higher-energy transition state. This makes Rh(III) complexes unusually slow to exchange ligands — a property exploited in stable catalytic systems and separation chemistry.

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Summary

Reference for rhodium oxidation states: Rh most commonly shows +3, with +1 critical in catalysis, and rarer +2, +4, +5, +6 states documented.

How it works

Click a tab — Oxidation States, Compounds, Electron Config, or Physical Props — to explore each section.
The Oxidation States panel lists every known state from −1 to +6 with stability notes and example species.
The Compounds panel shows key rhodium compounds with their formulas and assigned Rh oxidation state.
The Electron Config panel diagrams orbital filling and ionization steps from Rh to Rh3+.
The Physical Props panel provides atomic and material data for quick lookup.
Click any monospace table cell to copy its value to your clipboard.

Use cases

Students studying d-block transition metal chemistry and variable oxidation states.
Researchers working with rhodium catalysts in homogeneous or heterogeneous catalysis.
Chemistry teachers preparing material on platinum-group metals (PGMs).
Materials scientists selecting rhodium compounds for electrochemical or coating applications.
Anyone preparing for exams covering Period 5 transition metals or organometallic chemistry.

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Last updated: 2026-07-08 · Reviewed by Nham Vu