$Mn ^{2+}$ compounds are more stable than $Fe ^{2+}$ compounds towards oxidation to $+3$ state due to its completely half-filled stable configuration.
$Fe ^{2+}$ can easily acquire $+3$ oxidation state by losing one electron as it attains the stable configuration but 3rd ionisation energy for $Mn ^{2+}$ is very high as it is very difficult to loose one electron from stable configuration $\left(d^{5}\right)$ therefore, $Mn ^{2+}$ is more stable than $Fe ^{2+}$
