Why are the $E^o$ values for $Mn^{2+}/Mn$,$Zn^{2+}/Zn$,and $Ni^{2+}/Ni$ more negative than expected? The table below provides the values:
| $E^o_{Mn^{2+}/Mn}$ | $E^o_{Zn^{2+}/Zn}$ | $E^o_{Ni^{2+}/Ni}$ |
| :--- | :--- | :--- |
| $-1.18 \ V$ | $-0.76 \ V$ | $-0.25 \ V$ |

(N/A) The standard electrode potential $(E^o)$ of a metal is determined by the sum of the enthalpy of atomization,ionization enthalpy,and hydration enthalpy.
$1$. For $Mn^{2+}/Mn$: The $Mn^{2+}$ ion has a stable $d^5$ configuration. The high stability of the $d^5$ configuration makes the reduction of $Mn^{2+}$ to $Mn$ less favorable,resulting in a more negative $E^o$ value.
$2$. For $Zn^{2+}/Zn$: The $Zn^{2+}$ ion has a stable $d^{10}$ configuration. Similar to $Mn^{2+}$,the stability of the $d^{10}$ configuration makes the reduction of $Zn^{2+}$ to $Zn$ less favorable,leading to a more negative $E^o$ value.
$3$. For $Ni^{2+}/Ni$: The $Ni^{2+}$ ion has a very high negative hydration enthalpy,which compensates for the energy required for ionization and atomization,resulting in a more negative $E^o$ value than expected from the general trend.