Why `E^(0)` values for Mn, Ni and Zn are more negative than expected ?

Explanation

`E^(0)` values for Mn and Zn are more negative than expected, because `Mn^(2+)` and `Zn^(2+)` ions have extra stability owing to the presence of half - filled `d^(5)` and completely filled `d^(10)` configuration of d - subshell. The `-ve E^(0)` value for Ni is related to very high negative enthalpy of hydration of `Ni^(2+)` ion.