Four successive members of the first row of transition elements are listed below with atomic numbers. Which one of them is expected to have the highest $$E^\circ_{M^{3+}/M^{2+}}$$ value?

A higher $$E^\circ_{M^{3+}/M^{2+}}$$ value indicates a greater tendency for the $$M^{3+}$$ ion to be reduced to the $$M^{2+}$$ state. This depends on the relative stability of the electronic configurations and hydration enthalpies in aqueous solution.

Chromium: $$Cr^{3+}$$ ($$3d^3$$, stable $$t_{2g}^3$$ half-filled level) is more stable than $$Cr^{2+}$$. Thus, it has a negative reduction potential.

Manganese: $$Mn^{2+}$$ ($$3d^5$$, half-filled) is more stable than $$Mn^{3+}$$. This leads to a high positive value.

Iron: $$Fe^{3+}$$ ($$3d^5$$, half-filled) is more stable than $$Fe^{2+}$$. This makes the $$E^\circ$$ value lower than that of Manganese.

Cobalt: $$Co^{2+}$$ is much more stable than $$Co^{3+}$$ in aqueous solution because $$Co^{3+}$$ is a powerful oxidizing agent that can even oxidize water. This results in the highest reduction potential in the series.

Standard Reduction Potential Values: $$E^\circ_{Cr^{3+}/Cr^{2+}} = -0.41 \text{ V}$$, $$E^\circ_{Mn^{3+}/Mn^{2+}} = +1.57 \text{ V}$$, $$E^\circ_{Fe^{3+}/Fe^{2+}} = +0.77 \text{ V}$$, $$E^\circ_{Co^{3+}/Co^{2+}} = +1.97 \text{ V}$$