Question:
Why E° values for Mn, Ni and Zn are more negative than expected?
Solution:
Mn2+(3d5) and Zn2+(3d10) have half-filled and filled d orbitals which give them stability and therefore prefer to stay that way and not get reduced. As for Ni2+(3d8), it has
very high negative hydration enthalpy which gets balanced by first and second ionization enthalpy.