What is the most stable oxidation state of cobalt in water?

Co(II) — the pink [Co(H2O)6]2+ ion — is the thermodynamically stable form in aqueous solution. Co(III) salts are strong oxidizing agents and are reduced back to Co(II) in simple aqueous media unless stabilized by strong-field ligands.

Why are Co(III) complexes kinetically inert?

Co(III) is a d6 low-spin ion with a large crystal-field splitting energy. This gives a high activation barrier for ligand substitution, making Co(III) complexes exchange ligands millions of times more slowly than labile ions like Co(II).

What causes the color change from pink to blue in cobalt solutions?

The pink color comes from the octahedral [Co(H2O)6]2+ ion. Adding concentrated chloride shifts the equilibrium to the tetrahedral [CoCl4]2- ion, which is deep blue. The color change reflects the change in coordination geometry and ligand-field splitting.

Does cobalt have oxidation states beyond +2 and +3?

Yes. Cobalt can formally adopt -1, 0, +1, +4, and +5 states, though these are rare. Co(0) occurs in carbonyls like Co2(CO)8, Co(-I) in [Co(CO)4]-, and Co(+4)/Co(+5) only in exotic fluoride or oxide compounds under extreme conditions.

What is the electron configuration of Co2+ and Co3+?

Neutral cobalt is [Ar] 3d7 4s2. Co2+ loses both 4s electrons: [Ar] 3d7. Co3+ loses one additional 3d electron: [Ar] 3d6.

What is cobalt's role in vitamin B12?

Vitamin B12 (cobalamin) contains a Co(III) center coordinated in a corrin ring. The cobalt cycles between Co(III), Co(II), and Co(I) during its enzymatic action, making it one of the few biological uses of Co(I).

Cobalt Oxidation States

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

Explore cobalt oxidation states (+2, +3, and others), key compounds, solution colors, and coordination chemistry in one interactive reference card.

27 Co Cobalt

Cobalt — Element 27

Transition metal · Group 9 · Period 4 · Block d
Atomic mass: 58.933 u · Ground-state config: [Ar] 3d⁷ 4s²

Most stable: Co(II) Kinetically inert: Co(III) Rare: Co(-I, 0, +I, +IV, +V)

Oxidation State

+2 (Co²⁺)

Most common & thermodynamically stable in water

Electron Config

[Ar] 3d⁷

High-spin in weak fields; low-spin possible with strong-field ligands

Aqueous color

Pink / Rose

[Co(H₂O)₆]²⁺

Key Co(II) Compounds

Formula	Name	Color	Notes
CoCl₂	Cobalt(II) chloride	Blue (anhydrous)	Turns pink when hydrated; humidity indicator
CoCl₂·6H₂O	Cobalt(II) chloride hexahydrate	Pink	Common lab reagent; octahedral Co²⁺
CoSO₄	Cobalt(II) sulfate	Pink (hydrated)	Electroplating bath; feed supplement
CoO	Cobalt(II) oxide	Gray-black	Ceramic pigment; refractory material
Co(NO₃)₂	Cobalt(II) nitrate	Red	Catalyst precursor; deliquescent solid
[CoCl₄]^2-	Tetrachlorocobaltate(II)	Deep blue	Formed in conc. HCl; tetrahedral geometry

Coordination Geometry

✓Octahedral — most common; [Co(H₂O)₆]²⁺, [Co(NH₃)₆]²⁺
✓Tetrahedral — favored with large halide ligands; [CoCl₄]^2-
✓Square planar — rare for Co(II); requires very strong-field ligands

Stability & Redox

✓Stable in air and water under normal conditions
✓E°(Co³⁺/Co²⁺) = +1.92 V (aquo ions) — Co(III) is a strong oxidizer
✓Labile — ligand exchange is fast (microsecond timescale)

Oxidation State

+3 (Co³⁺)

Unstable in simple salts; dramatically stabilized by strong-field ligands

Electron Config

[Ar] 3d⁶

Low-spin d⁶ with strong-field ligands; kinetically inert

Aqueous color

Blue-green

[Co(H₂O)₆]³⁺ (unstable)

Key Co(III) Compounds

Formula	Name	Color	Notes
Co₂O₃	Cobalt(III) oxide	Black	Forms above 400 °C; decomposes to CoO + O₂
CoF₃	Cobalt(III) fluoride	Tan / brown	Strong fluorinating agent; reacts violently with water
[Co(NH₃)₆]³⁺	Hexaamminecobalt(III)	Yellow	Classic Werner complex; kinetically inert
[Co(en)₃]³⁺	Tris(ethylenediamine)cobalt(III)	Orange	Chiral; used in resolution of racemates (Werner 1911)
[Co(CN)₆]^3-	Hexacyanocobaltate(III)	Colorless/pale	Very large Δ, strongly stabilized; non-toxic unlike Fe analog
Vitamin B₁₂	Cobalamin	Red	Co(III) in corrin ring; biologically cycles Co(I/II/III)

Coordination Geometry

✓Octahedral — exclusively; Co(III) d⁶ low-spin fills t_2g completely
✓No tetrahedral Co(III) complexes are known to be stable
✓Ligand substitution half-life: hours to years (inert)

Stability & Redox

✓Aquo ion [Co(H₂O)₆]³⁺ oxidizes water — unstable in solution
✓Strong-field ligands lower E° into the stable window (e.g., CN^-: E° ≈ -0.83 V)
✓CFSE for low-spin d⁶ octahedral = -2.4Δ_o (very large)

Beyond the dominant +2 and +3 states, cobalt can adopt several uncommon formal oxidation states in specialized environments.

State	d-count	Example compound	Environment
-I	d¹⁰	[Co(CO)₄]^-	Strong π-acceptor CO ligands; tetrahedral
0	d⁹	Co₂(CO)₈	Metal carbonyls; hydroformylation catalyst precursor
+1	d⁸	CoCl(PPh₃)₃	Phosphine complexes; B₁₂ enzymatic intermediate (Co^I)
+2	d⁷	CoCl₂, [Co(H₂O)₆]²⁺	Most common; stable in water
+3	d⁶	[Co(NH₃)₆]³⁺	Stable with strong-field ligands; kinetically inert
+4	d⁵	CoF₄ (matrix)	Exotic; only in fluoride matrices or high-pressure oxides
+5	d⁴	CoO₄^3- (theoretical)	Extremely rare; proposed in some oxide structures

Note on formal vs. physical oxidation state: In organometallic and carbonyl complexes, formal oxidation states can differ substantially from the actual electron density on cobalt. Co(0) and Co(-I) are assigned by convention but the metal retains significant electron density due to back-bonding from CO.

Quick comparison of the two dominant cobalt oxidation states across key chemical properties.

Property	Co(+2)	Co(+3)
d-electron count	d⁷	d⁶
Spin state (weak field)	High-spin	High-spin (unstable)
Spin state (strong field)	Low-spin possible	Low-spin (very stable)
Preferred geometry	Octahedral or tetrahedral	Octahedral only
Aquo ion color	Pink ([Co(H₂O)₆]²⁺)	Blue-green ([Co(H₂O)₆]³⁺)
Aqueous stability	Stable	Oxidizes water; unstable
Ligand substitution	Labile (μs timescale)	Inert (hours–years)
E° (Co³⁺/Co²⁺)	+1.92 V (aquo); decreases with strong-field ligands to <0 V
Common precipitate	Co(OH)₂ (pink/blue)	Co(OH)₃ (brown)
Biological role	Intermediate in B₁₂ cycle	Resting state in cobalamin

Summary

Explore cobalt oxidation states (+2, +3, and others), key compounds, solution colors, and coordination chemistry in one interactive reference card.

How it works

Click any oxidation state tab to load its detail card.
The card shows electron configuration, key compounds, solution color, and coordination geometry.
Use the compound table to compare formulas, names, and colors side by side.
Switch to the "Color Guide" tab to see a visual swatch for each aqueous ion.
The "All States" tab gives a quick-comparison grid of every known oxidation state.

Use cases

Reviewing cobalt chemistry for general or inorganic chemistry courses.
Identifying cobalt compounds by their characteristic solution colors in lab.
Understanding why Co(III) complexes are kinetically inert (ligand-field stabilization).
Comparing Co(II) vs Co(III) stability in aqueous vs coordinated environments.
Preparing for qualitative analysis tasks involving transition metal ions.
Cross-referencing oxidation states when interpreting spectroscopic data.

Frequently Asked Questions

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Last updated: 2026-05-28 · Reviewed by Nham Vu