What is the most common oxidation state of nickel?

Nickel most commonly exists as Ni²⁺ (+2 oxidation state). Its [Ar] 3d⁸ configuration after losing two 4s electrons is relatively stable, and +2 compounds are abundant — NiSO₄, NiCl₂, NiO, and Ni(OH)₂ are all familiar examples. Aqueous Ni²⁺ solutions are characteristically pale green.

What electron configuration does Ni²⁺ have?

Neutral nickel is [Ar] 3d⁸ 4s². The two 4s electrons are removed first on ionization to give Ni²⁺: [Ar] 3d⁸. This leaves eight d-electrons and is the basis for nickel's many square-planar and octahedral d⁸ complexes.

Where does the +3 oxidation state appear?

Ni³⁺ appears in nickel oxyhydroxide (NiOOH), the charged form of the cathode material in nickel-metal hydride (NiMH) and nickel-cadmium (NiCd) batteries. During discharge, Ni³⁺ is reduced to Ni²⁺; during charge, Ni²⁺ is oxidized back to Ni³⁺. NiF₃ is another example isolated under strongly oxidizing conditions.

What is Ni(CO)₄ and why is nickel in the 0 oxidation state?

Tetracarbonylnickel, Ni(CO)₄, is an organometallic compound formed by passing CO over finely divided nickel metal. Each CO ligand donates electron density to Ni while accepting back-donation from filled d-orbitals (π back-bonding). Because the four CO ligands are neutral, nickel retains a formal oxidation state of 0. Ni(CO)₄ is used in the Mond process to purify nickel metal.

Does nickel have negative oxidation states?

Yes, Ni(−I) and Ni(−II) have been characterized in carbonyl cluster complexes and certain organometallic compounds under reducing conditions. For example, [Ni(CO)₄]²⁻ formally contains Ni(−II). These states are rare and not encountered in ordinary chemistry.

Why can nickel adopt multiple oxidation states?

Nickel is a d-block transition metal with partially filled 3d orbitals. The relatively small energy differences between successive ionization energies for d electrons, combined with stabilization through ligand-field effects in complexes, allow multiple oxidation states to be accessible. Crystal field stabilization energy (CFSE) plays a key role in determining which state is favored for a given ligand set.

Nickel Oxidation States

Name: Nickel Oxidation States
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Author: Nham Vu

Reference for all nickel oxidation states: +2 dominates, with +3 in NiMH batteries and +4 in NiO₂. Includes d-electron configs, example compounds, and colors.

Atomic # 28 Ni Nickel

Atomic Mass

58.6934 u

Group

10 (VIIIB)

Period

Block

d-block

Electronegativity

1.91 (Pauling)

Common States

0, +2, +3, +4

Nickel is a Period 4 d-block metal. Its ground-state configuration is [Ar] 3d⁸ 4s². Losing both 4s electrons gives the dominant +2 state (d⁸). Higher states +3 and +4 require removal of 3d electrons and are stabilized by strongly oxidizing ligands or electrode conditions. Negative states exist in carbonyl cluster chemistry.

State	d-electrons	Config (after Ar)	Color (aq.)	Stability	Notes
−2	d¹²	3d¹⁰ 4s²	—	Exotic	Formal state in carbonylnickelate anions such as [Ni(CO)₄]²⁻. Requires strongly reducing, inert conditions.
−1	d¹¹	3d¹⁰ 4s¹	—	Exotic	Found in some carbonyl cluster compounds. Not isolable as simple aquo ion.
0	d¹⁰	3d¹⁰	Colorless (gas)	Organometallic	Ni(CO)₄ (tetracarbonylnickel); Ni(0) phosphine complexes used in cross-coupling catalysis. Industrially important in the Mond process.
+1	d⁹	3d⁹	—	Uncommon	Paramagnetic (one unpaired electron). Seen in some phosphine and N-heterocyclic carbene complexes. Not stable in aqueous solution.
+2	d⁸	3d⁸	Pale green	Dominant	Most stable aqueous state. NiSO₄ (green), NiCl₂ (yellow-green), Ni(OH)₂ (green), NiO (gray-green). Forms octahedral, tetrahedral, and square-planar complexes.
+3	d⁷	3d⁷	Black / dark	Battery use	NiOOH (nickel oxyhydroxide) — cathode material in NiMH and NiCd batteries. NiF₃ isolable under strongly oxidizing conditions. Paramagnetic (3 unpaired electrons).
+4	d⁶	3d⁶	Black	Rare	NiO₂ — used as battery electrode material; potent oxidizer. Also BaNiO₃ and some fluoride complexes. Strong oxidants required to reach this state.

Redox context: The Ni²⁺/Ni⁰ standard reduction potential is −0.257 V. The Ni³⁺/Ni²⁺ couple in alkaline solution (as in NiOOH/Ni(OH)₂) is approximately +0.49 V. These potentials underpin both hydrometallurgical nickel refining and the electrochemistry of rechargeable Ni-based batteries.

Nickel forms compounds across states 0, +2, +3, and +4. The vast majority of commercially relevant compounds feature Ni²⁺. Higher states are typically found in battery materials or fluoride chemistry.

Compound	Formula	Ni State	Notes
Tetracarbonylnickel	Ni(CO)₄	0	Colorless toxic gas; Mond process intermediate for purifying Ni metal; decomposes above 43 °C releasing pure Ni.
Bis(1,5-cyclooctadiene)nickel	Ni(cod)₂	0	Yellow solid; versatile Ni(0) catalyst precursor in cross-coupling (Suzuki, Kumada, Negishi) reactions.
Nickel(II) Oxide	NiO	+2	Gray-green solid; used in ceramics, battery electrodes, and as a catalyst precursor.
Nickel(II) Sulfate	NiSO₄	+2	Green crystalline solid; widely used in nickel electroplating baths and as a reagent.
Nickel(II) Chloride	NiCl₂	+2	Yellow-green solid; hygroscopic; common laboratory reagent and catalyst precursor.
Nickel(II) Hydroxide	Ni(OH)₂	+2	Green precipitate; forms on adding base to Ni²⁺ solution; precursor to NiOOH in batteries.
Nickel(II) Sulfide	NiS	+2	Black solid; occurs in millerite ore; semiconductor applications.
Nickel(II) Carbonate	NiCO₃	+2	Light green solid; used in catalysts and as a pigment precursor.
Nickel(II) Nitrate	Ni(NO₃)₂	+2	Green deliquescent solid; laboratory reagent; decomposes to NiO on heating.
Nickel(II) Acetate	Ni(CH₃COO)₂	+2	Green solid; catalyst in organic synthesis; used in metal-organic frameworks.
Nickel(II) Bromide	NiBr₂	+2	Yellow solid; used as precursor to Ni(0) complexes in organometallic chemistry.
Nickelocene	Ni(C₅H₅)₂	+2	Green paramagnetic metallocene; notable for its 20-electron count (not 18).
Nickel Oxyhydroxide	NiOOH	+3	Black solid; cathode material in NiMH and NiCd rechargeable batteries; oxidizes Ni(OH)₂ on charging.
Nickel(III) Fluoride	NiF₃	+3	Dark solid; isolable under strongly oxidizing fluorinating conditions; strong oxidizer.
Nickel(IV) Oxide	NiO₂	+4	Black solid; powerful oxidant; used as electrode material; reacts vigorously with reducing agents.
Barium Nickelate	BaNiO₃	+4	Perovskite-structure oxide; Ni in rare +4 state; studied in solid-state and materials chemistry.

Ground State Configuration

Core

Core (part of [Ar])

Valence — 8 electrons; 2 unpaired (t₂g⁶ eₘ²)

Outer shell — lost first on ionization

Notation

Full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁸ 4s²

Noble gas: [Ar] 3d⁸ 4s²

Ni²⁺ ion: [Ar] 3d⁸

Ni³⁺ ion: [Ar] 3d⁷

Ionization Steps

[Ar] 3d⁸ 4s²

Neutral atom — 10 valence electrons

Ni⁺

[Ar] 3d⁸ 4s¹

Remove one 4s electron (IE₁ ≈ 737 kJ/mol)

Ni²⁺

[Ar] 3d⁸

Remove last 4s electron (IE₂ ≈ 1753 kJ/mol) — most stable aqueous ion

Ni³⁺

[Ar] 3d⁷

Remove a 3d electron (IE₃ ≈ 3395 kJ/mol) — battery cathode state

Ni⁴⁺

[Ar] 3d⁶

Remove another 3d electron (IE₄ ≈ 5300 kJ/mol) — strong oxidant only

d⁸ square-planar tendency: Ni²⁺ with a d⁸ configuration has a strong crystal-field stabilization energy in square-planar geometry — larger than in tetrahedral for strong-field ligands. This is why Ni²⁺ forms square-planar complexes with CN⁻, phosphines, and similar ligands, while weak-field ligands give octahedral or tetrahedral geometry.

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Summary

Reference for all nickel oxidation states: +2 dominates, with +3 in NiMH batteries and +4 in NiO₂. Includes d-electron configs, example compounds, and colors.

How it works

Click a tab — Oxidation States, Compounds, Electron Config, or Physical Props — to navigate sections.
The Oxidation States panel shows every known state with d-electron count, stability badge, and color indicator.
The Compounds panel lists real Ni compounds grouped by oxidation state with formulas and notes.
The Electron Config panel shows orbital filling for Ni(0) and key ions Ni²⁺ and Ni³⁺.
The Physical Props panel lists atomic and material data for quick reference.
Click any monospace table cell to copy its content to the clipboard.

Use cases

Students studying d-block transition metal chemistry and variable oxidation states.
Chemistry teachers preparing lessons on nickel coordination chemistry or battery electrochemistry.
Researchers working with nickel catalysts or nickel-based electrode materials.
Engineers evaluating NiMH battery chemistry needing Ni²⁺/Ni³⁺ redox context.
Anyone preparing for exams covering Period 4 transition metals.

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