Chromium, cobalt, vanadium and manganese chemistry

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COBALT

Cobalt exists in two stable oxidation states, +2 and +3.

Acidic or neutral solutions:  [Co(H2O)6]2+ pink

Alkaline solution:    [Co(H2O)4(OH)2] blue (s)

COBALT A2 Transition Metals Variable Oxidation States

Oxidation of Co2+ by air or H2O2 in ammoniacal solution through a sequence of reactions:

1. Ammonia acting as a base:
[Co(H2O)6]2+  +  2OH−  →  [Co(H2O)4(OH)2] + 2H2O
[Co(H2O)6]2+  +  2NH3  →  [Co(H2O)4(OH)2] +2NH4+
(pink solution)                 (blue (s))

2. Ligand exchange reaction (excess NH3):
[Co(H2O)4(OH)2]  + 6NH3  → [Co(NH3)6]2+  +  2OH−  + 4H2O
(blue (s))                            (yellow/straw solution)

3. Oxidation reaction ( by oxygen in air or by H2O2):

Utilising the following half-equations for the oxidising agent:

O2  + 2H2O + 4e− →  4OH−
H2O2  + 2e− →  2OH−

4[Co(NH3)6]2+  + O2  + 2H2O  →  4[Co(NH3)6]3+  +  4OH−
(yellow/straw solution)            (dark brown solution)

2[Co(NH3)6]2+  +   H2O2         →  2[Co(NH3)6]3+  +  2OH−
(yellow/straw solution)            (dark brown solution)

  • Similarly, ethane-1,2-diamine may be complexed with Co2+ and oxidised by oxygen in air to Co3+ complex.

4[Co(H2NCH2CH2NH2)3]2+  + O2  + 2H2O  →  4[Co(H2NCH2CH2NH2)3]3+  +  4OH−

Ligand substitution with HCl

[Co(H2O)6]2+  +  4Cl− ⇌  [CoCl4]2− + 6H2O     ΔH +ve (endothermic)
(pink solution)              (blue solution)

The larger sized chloride ion ligand leads to a change in coordination number, complex shape (octahedral to tetrahedral), colour, and cationic to anionic form, without any change in oxidation state.
In accordance with Le Chatelier’s principle, dilution and/or cooling the blue complex shifts the equilibrium position to the left, promoting formation of the pink complex.

Oxidation of Iodide by [Co(H2O)6]3+

2[Co(H2O)6]3+  +  2I− ⇌ 2[Co(H2O)6]2+ + I2

Summary of key reaction pathways for cobalt

COBALT III latest TM Ligand substitution and variable oxidation states

[Co(NH3)6]2+ + CO32−  → CoCO3 + 6H2O
(pink solution)               (purple/pink solid)

[Co(H2O)6]2+  + 6NH3  → [Co(NH3)6]2+  +  6H2O
(pink solution)               (yellow/straw solution)

2[Co(NH3)6]2+  +H2O2  →   2[Co(NH3)6]3+  +  2OH−
(yellow/straw solution)    (dark brown solution)

Let en represent the bidentate ethane-1,2-diamine ligand (H2NCH2CH2NH2)

[Co(NH3)6]2+ +   3en    \rightleftharpoons   [Co(en)3]2+ + 6NH3
(yellow/straw solution)  (yellow/orange solution)

[Co(H2O)6]2+  +  4Cl−   ⇌  [CoCl4]2− + 6H2O
(pink solution)                (blue solution)

cobalt2 A2 Transition Metals Variable Oxidation States

Reaction 1 uses reagent NH3:      [Co(H2O)6]2+ + 6NH3   → [Co(NH3)6]2+ + 6H2O

Reaction 2 uses reagent H2O2:     2[Co(NH3)6]2+ + H2O2 → 2[Co(NH3)6]3+ + 2OH−

Reaction 3 uses reagent HCl:       [Co(H2O)6]2+ + 4HCl    → [CoCl4]2− + 6H2O + 4H+

Reaction 4 uses reagent Na2CO3: [Co(H2O)6]2+ + CO32−  → CoCO3 + 6H2O

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