Why E° values for Mn, Ni and Zn are more negative than expected?

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dandf-blockelements

#1

Negative values of ${{Mn}^{2+}}$ and ${{Zn}^{2+}}$ are related to stabilities of half-filled and completely filled configuration respectively. But for ${{Ni}^{2+}}$, E° value is related to the highest negative enthalpy of hydration.
Hence, E° values of Mn, Ni and Zn are more negative than expected.