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Poutnik should have added that the author of the mentioned sentence is clearly wrong. But the author would have been right if he would have replaced $\ce{H2O}$ by $\ce{NH3}$. Matter of fact, $\ce{[Co(NH3)6]^{2+}}$ is easily oxidized in $\ce{[Co(NH3)6]^{3+}}$ for example by atmospheric $\ce{O2}$ molecules. The standard electrode potential for the reaction $\... 4 The author of the quoted question is obviously mistaken. The reduced form$\ce{[Co(H2O)6]^2+}$does not manifest significant reducing abilities and is oxidized with great difficulties only. The oxidized form$\ce{[Co(H2O)6]^3+}$is very strong oxidizer. The respective ox/red pair has the standard redox potential$E^{\circ}(\ce{Co^3+}/\ce{Co^2+})=\pu{+1.82 ...