Ferric ion with ${\mathrm{d}}^5$ configuration is more stable than ferrous ion.

Explanation:

${\mathrm{Fe}}^{2+}$ has the atomic formula $\left[\mathrm{Ar}\right] 3{\mathrm{d}}^6 4{\mathrm{s}}^0$.

${\mathrm{Fe}}^{3+}$ has the atomic formula $\left[\mathrm{Ar}\right] 3{\mathrm{d}}^5 4{\mathrm{s}}^0$.

Accordingly, ${\mathrm{Fe}}^{3+}$ is more stable because it achieves a more stable half-filled d subshell, which is more stable due to increased energy exchange and symmetry.