The correct order of ${\mathrm{E}}_{{\mathrm{M}}^{2+}/\mathrm{M}}^{\mathrm{o}}$ values with negative sign for the four successive elements Cr, Mn, Fe and Co is

• Usually across the first transition series, the negative values for standard electrode potential decrease except for Mn due to stable d5-configuration.
• The electronic configuration of various elements in Mn²⁺ state is:

          Cr- [Ar] 3d⁴

          Mn-Cr- [Ar] 3d⁵     -half filled- most stable.

          Fe-Cr- [Ar] 3d⁶

          Co-Cr- [Ar] 3d⁷

• So, based on above facts the correct order is: Mn> Cr> Fe> Co.