Why does the Co³⁺/Co²⁺ couple have such a high reduction potential?

Cobalt is changing from $\mathrm{(3d)^6}$ to $\mathrm{(3d)^7}$ electronic configuration. What's so stabilizing about that? Just by reduction potentials this couple is more oxidising than hydrogen peroxide and that just sounds amazing and crazy at the same time.




**The electronic configuration has nothing to do with it.** The reduction potentials of $\ce{Ni^3+}/\ce{Ni^2+}$, $\ce{Cu^3+}/\ce{Cu^2+}$ and $\ce{Zn^3+}/\ce{Zn^2+}$, if they have been/could be measured, would be even greater.


The reduction potential for $\ce{M^3+}/\ce{M^2+}$ is most dependent upon the third ionisation energy. If $I\_3$ is large then it will be difficult to oxidise $\ce{M^2+}$ to $\ce{M^3+}$ (hence a high reduction potential). Likewise if $I\_3$ is small then the reduction potential will be smaller.


$$\begin{array}{c|cc} \hline
\text{Metal} & I\_3\text{ / eV} & E^\circ(\ce{M^3+}/\ce{M^2+})\text{ / V} \\ \hline
\ce{Sc} & 24.76 & (-2.6) \\
\ce{Ti} & 27.48 & (-1.1) \\
\ce{V} & 29.31 & -0.26 \\
\ce{Cr} & 30.96 & -0.41 \\
\ce{Mn} & 33.67 & +1.60 \\
\ce{Fe} & 30.65 & +0.77 \\
\ce{Co} & 33.50 & +1.93 \\
\ce{Ni} & 35.16 & (+4.2) \\
\ce{Cu} & 36.94 & (+4.6) \\
\ce{Zn} & 39.72 & (+7.0) \\ \hline
\end{array}$$


[![enter image description here](https://i.stack.imgur.com/c4qeE.png)](https://i.stack.imgur.com/c4qeE.png)


Data from Weller et al. *Inorganic Chemistry* 6ed; Johnson *Some Thermodynamic Aspects of Inorganic Chemistry*. Numbers in parentheses indicate predicted values. Graph by me in Microsoft Office 365.


So your question is essentially: why is $I\_3$ of $\ce{Co}$ so "large"?


Going across the 3d block, effective nuclear charge increases which leads to the **general** trend of IE3 and $E^\circ(\ce{M^3+}/\ce{M^2+})$ increasing.


Any stabilisation derived from "special" electronic configurations e.g. the half-filled subshell in $\ce{Fe^3+}$, or the ligand-field stabilisation energy of $\ce{Cr^2+}$, merely lead to *relatively small deviations from the general trend*.


Moral of the story is: look at the big picture first. Those $\mathrm{d^5}, \mathrm{d^{10}}, \cdots$ configurations *can* be relevant, but they *aren’t* responsible for the overall trend, and certainly *shouldn’t* be the first thing you consider when trying to rationalise this set of data.


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