What are the most common oxidation states of iridium?

In most practical chemistry, iridium is encountered as Ir(III) and Ir(IV). Ir(III) dominates classical coordination chemistry (e.g., Ir(NH3)6 3+ and Crabtree's catalyst precursors), while Ir(IV) appears in IrO2, IrCl4, and electrocatalysis. Ir(I) is important in organometallic chemistry, including Vaska's compound (IrCl(CO)(PPh3)2).

Why can iridium reach the unusually high +9 oxidation state?

IrO4+ (formally Ir+9) has been detected in gas-phase mass spectrometry experiments, but it is not an isolable compound. The high states (+6, +7, +8, +9) are accessible because iridium's large number of d-electrons and strong metal–oxygen bonding allow very high oxidation. The highest confirmed bulk compound is IrF6 (Ir+6), and IrO4 (Ir+8) has been observed spectroscopically.

What is the electron configuration of iridium?

The ground-state configuration is [Xe] 4f14 5d7 6s2 (atomic number 77). When oxidized to Ir3+, three electrons are removed (primarily from 5d and 6s), giving [Xe] 4f14 5d6. Ir4+ loses one more to give [Xe] 4f14 5d5. Relativistic contraction of the 6s orbital influences bond lengths and stabilizes unusual oxidation states.

How does iridium compare to its neighbour osmium in terms of oxidation states?

Osmium (Z=76) is directly to the left of iridium and reaches even higher states — OsO4 with Os(+8) is a well-known volatile compound. Iridium's highest confirmed bulk oxidation state is +6 (IrF6). Both elements show a much wider range than their lighter 4d congeners (rhodium and ruthenium) due to relativistic effects in the 5d series.

What is Crabtree's catalyst and what oxidation state does it use?

Crabtree's catalyst is [Ir(cod)(PCy3)(py)]PF6, where iridium is in the +1 state. It is a powerful homogeneous hydrogenation catalyst that can reduce hindered olefins even tetrasubstituted ones under mild conditions. The active catalytic species cycles between Ir(I) and Ir(III) during the catalytic cycle.

Is iridium a rare element?

Yes. Iridium is one of the rarest stable elements in Earth's crust (abundance ~0.001 ppm). Most commercial iridium comes as a byproduct of platinum and nickel mining. Despite its rarity, its extreme corrosion resistance, hardness, and catalytic properties make it commercially significant.

Iridium Oxidation States

Name: Iridium Oxidation States
Availability: InStock
Author: Nham Vu

Reference page covering all known oxidation states of iridium (Ir, element 77), with the common +3 and +4 states explained through electron configuration, ionization context, and key compounds.

Atomic # 77 Ir Iridium

Atomic Mass

192.217 u

Group

9 (VIIIB)

Period

Block

d-block

Electronegativity

2.20 (Pauling)

Oxidation States

−3 to +9

Iridium (Ir, Z = 77) has the electron configuration [Xe] 4f¹⁴ 5d⁷ 6s² and exhibits one of the broadest oxidation state ranges in the periodic table — from −3 to +9. In practice, +3 and +4 account for the majority of isolable compounds; +1 is critical in organometallic and homogeneous catalysis. Very high states (+6 to +9) are observed mainly in fluorides and in gas-phase species. Relativistic effects on the 5d/6s shells contribute to the unusual breadth.

Oxidation State	Stability	Notes
−3	Rare / exotic	amber
−1	Rare / exotic	amber
0	Elemental only	gray
+1	Well-established	blue
+2	Uncommon	amber
+3	Stable — dominant	green
+4	Stable	green
+5	Moderate	amber
+6	Isolable	amber
+7	Extreme	red
+8	Spectroscopic	red
+9	Gas-phase only	red

Ionization Energies (first three)
IE₁ = 880 kJ/mol | IE₂ = 1600 kJ/mol | IE₃ = 2400 kJ/mol
Iridium's high IE₁ relative to lighter congeners reflects 5d–6s relativistic stabilization. The d⁶ configuration of Ir³⁺ confers strong crystal-field stabilization energy, making +3 kinetically very inert in octahedral environments.

Iridium compounds span a wide range of oxidation states, though +3 and +4 account for most commercial and synthetic applications. Ir(I) organometallics are central to homogeneous catalysis; Ir(III) cyclometalated complexes are the emitters of choice in OLED devices.

Compound	Formula	Ir State	Notes
Vaska's Compound	IrCl(CO)(PPh₃)₂	+1	Classic Ir(I) complex that undergoes reversible oxidative addition of O2, H2, and other small molecules. Foundation of organometallic reactivity studies.
Crabtree's Catalyst	[Ir(cod)(PCy₃)(py)]PF₆	+1	Powerful homogeneous hydrogenation catalyst for hindered olefins. Active species cycles Ir(I)/Ir(III) during catalysis.
Iridium(III) Chloride	IrCl₃	+3	Starting material for most synthetic Ir(III) chemistry. Insoluble form is kinetically inert; the hydrate is more reactive.
Hexaammineiridium(III) Chloride	[Ir(NH₃)₆]Cl₃	+3	Classic Ir(III) coordination complex. Demonstrates the kinetic inertness of d6 octahedral iridium.
fac-Tris(2-phenylpyridine)Ir	fac-Ir(ppy)₃	+3	Benchmark phosphorescent OLED emitter (green). Strong spin–orbit coupling at Ir promotes triplet emission.
Iridium(IV) Oxide	IrO₂	+4	Thermally stable semiconductor; major industrial electrocatalyst for oxygen evolution reaction (OER) in water splitting and chlor-alkali cells.
Iridium(IV) Chloride	IrCl₄	+4	Hydrolyzes readily; used as a laboratory precursor for Ir(IV) chemistry. Less common than IrCl3 in practice.
Iridium(III) Oxide	Ir₂O₃	+3	Less stable than IrO2; can be formed by controlled oxidation of iridium metal at moderate temperatures.
Iridium Hexafluoride	IrF₆	+6	Volatile yellow solid (bp 53 °C); highest confirmed oxidation state in an isolable iridium compound. Powerful fluorinating agent.
Iridium Pentafluoride	IrF₅	+5	Tetrameric in the solid state (Ir4F20). Reactive solid that hydrolyzes in moist air.
Sodium Hexachloroiridate(IV)	Na₂IrCl₆	+4	Soluble Ir(IV) salt widely used as a convenient laboratory source of iridium. Starting material for many coordination compounds.
Iridium Tetroxide	IrO₄	+8	Observed only by matrix-isolation spectroscopy; not isolable as a bulk compound. Analogous to OsO4 but far more reactive.

Ground State Configuration

Lanthanide core — fully filled

Valence — 7 of 10 filled

Valence — removed early on ionization

Notation

Full: [Xe] 4f¹⁴ 5d⁷ 6s²

Ir³⁺ ion: [Xe] 4f¹⁴ 5d⁶ — d⁶ low-spin

Ir⁴⁺ ion: [Xe] 4f¹⁴ 5d⁵ — d⁵ low-spin

Ir¹⁺ ion: [Xe] 4f¹⁴ 5d⁸ — d⁸ square planar preference

Ionization / Key Ion Steps

[Xe] 4f¹⁴ 5d⁷ 6s²

Neutral atom — 9 valence electrons (5d+6s)

Ir¹⁺

[Xe] 4f¹⁴ 5d⁸

Remove 6s electrons first; gives d8 — catalysis

Ir²⁺

[Xe] 4f¹⁴ 5d⁷

d7 paramagnetic — uncommon, stabilized by strong-field ligands

Ir³⁺

[Xe] 4f¹⁴ 5d⁶

d6 low-spin — kinetically inert, OLED emitters, most stable in coordination chemistry

Ir⁴⁺

[Xe] 4f¹⁴ 5d⁵

d5 — IrO2 electrocatalyst; also found in IrF4, IrCl4

Ir⁶⁺

[Xe] 4f¹⁴ 5d³

d3 — highest confirmed in isolable compound IrF6

Why is Ir(III) so kinetically inert? Ir³⁺ has a d⁶ low-spin octahedral configuration with no electrons in eg* antibonding orbitals and a large crystal-field splitting energy (5d >> 3d). Ligand substitution requires either associative or dissociative pathways past a large activation barrier. This inertness is exploited in OLED emitters (fac-Ir(ppy)₃) and in probing reaction mechanisms with configurationally stable complexes.

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Summary

Reference page covering all known oxidation states of iridium (Ir, element 77), with the common +3 and +4 states explained through electron configuration, ionization context, and key compounds.

How it works

Click a tab — Oxidation States, Compounds, Electron Config, or Physical Props — to explore each area.
The Oxidation States panel lists every confirmed oxidation state with stability ratings and context notes.
The Compounds panel shows key iridium compounds, their formulas, and iridium oxidation state assignment.
The Electron Config panel diagrams orbital filling for neutral Ir and the common Ir3+/Ir4+ ions.
The Physical Props panel provides atomic and material data for quick reference.
Click any monospace table cell to copy its value to the clipboard.

Use cases

Students studying d-block transition metals and variable oxidation states.
Chemistry teachers preparing lessons on platinum-group element chemistry.
Researchers working on iridium catalysts for C–H activation or water oxidation.
Materials scientists studying iridium oxide (IrO2) electrodes for electrolysis.
Anyone preparing for chemistry exams covering Period 6 transition metals.
Inorganic chemists needing a quick reference for Ir compound oxidation state assignments.

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Last updated: 2026-07-23 · Reviewed by Nham Vu