Which ion is more stable in aqueous solution; Co2+, or Co3+? [closed]








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We know $\ce{Co^{3+}}$ is more stable in complex compounds than $\ce{Co^{2+}}$ is in the same. I am told that for aqueous solution, $\ce{Co^{2+}}$ is more stable than $\ce{Co^{3+}}$, however in aqueous solution too, $\ce{Co^{2+}}$ will have to form complex compounds in water so $\ce{Co^{3+}}$ should still be more stable???




Just because a metal can form complexes in different oxidation states with a given ligand, it does not follow that the complexes are equally stabilized in the different oxidation states by the same ligand. We often find that the relative stability of different oxidation states depends on the ligand environment. Copper, for instance, is stable only as copper(II) when solvated by water alone, but copper(I) becomes a stable oxidation state with a softer-base ligand such as thiourea or even chloride ion (see [here](https://chemistry.stackexchange.com/a/160317/17175)).


In the case of cobalt, ammonia is especially suitable for stabilizing cobalt(III) but water is not so good, so we find that with water solvation cobalt(III) is highly oxidizing and cobalt(II) is favored instead, whereas with ammonia complexation cobalt(III) is more stable relative to cobalt(II).


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