For the first row transition metals,the $E^{\Theta }$ values are:

$E^{\Theta }$	$V$	$Cr$	$Mn$	$Fe$	$Co$	$Ni$	$Cu$
$(M^{2+}/M)$	$-1.18$	$-0.91$	$-1.18$	$-0.44$	$-0.28$	$-0.25$	$+0.35$

Explain the irregularity in the above values.

(A) The $E^{\Theta } (M^{2+}/M)$ values are not regular because they depend on the sum of the enthalpy of sublimation,the enthalpy of ionization,and the enthalpy of hydration.
$1$. The irregular variation in ionization enthalpies $(\Delta_{i} H_{1} + \Delta_{i} H_{2})$ plays a significant role in these values.
$2$. The enthalpy of sublimation is also a key factor,which is relatively much lower for elements like $Mn$ and $V$,contributing to their more negative $E^{\Theta }$ values.
$3$. The stability of the $d^5$ configuration in $Mn^{2+}$ and the high hydration energy of $Cu^{2+}$ (which makes its $E^{\Theta }$ positive) are also critical factors.