Different Trends in d-block elements

Trends in the M²⁺/M Standard Electrode Potentials

There is no regular trend in the E⁰ (Mn²⁺/M) values. This is because their ionization enthalpies (I1 + IE2) and sublimation enthalpies do not show any regular trend.
The general trend towards less negative E⁰ values along the series is due to the general increase in the sum of first and second ionization enthalpies.
Copper shows a unique behaviour in the series as it is the only metal having positive value for E⁰. This explains why is does not liberate H2 gas from acids. It reacts only with the oxidizing acids (HNO₃ and H₂SO₄) which are reduced. The reason for positive E⁰ value for copper is that the sum of enthalpies of sublimation and ionization is not balanced by hydration enthalpy.
The values of E⁰ for Mn, Ni and Zn are more negative than expected from the general trend. This is due to greater stability of half-filled d-subshell (d⁵) in Mn²⁺, and completely filled d-subshell (d¹⁰) in Zn²⁺. The exceptional behaviour of Ni towards E⁰ value from the regular trend is due to its high negative enthalpy of hydration.

A very low value for E⁰ (Sc³⁺/Sc²⁺) reflects the stability of Sc3+ ion which has a noble gas configuration.
The highest value for Zn is on account of very high stability of Zn²⁺ ion with d¹⁰ configuration. It is difficult to remove an electron from it to change it into +3 states.
The comparatively high value of E⁰ (Mn³⁺/Mn²⁺) shows that Mn²⁺ is very stable which is on account of stable d^s configuration of Mn²⁺.
The comparatively low value of E⁰ (Fe³⁺/Fe²⁺) is on account of extra stability of Fe³⁺ (d^s), i.e., low third ionization enthalpy of Fe.
The comparatively low value for V is on account of the stability of V²⁺ ion due to its half-filled t³_2g configuration.