In general, the stability of \(+2\) oxidation state in first transition series decreases from left to right due to increase in the sum of first and second ionisation energies. However \(\mathrm{Mn}^{2+}\) is more stable due to half filled d-orbitals \(\left(3 d^5\right)\) and \(\mathrm{Zn}^{2+}\) is more stable due to completely filled \(d\)-orbitals \(\left(3 d^{10}\right)\).