The highly positive $E^\circ$ value for the $Mn^{3+}/Mn^{2+}$ reduction couple indicates a strong tendency for the $Mn^{3+}$ ion to accept an electron and get reduced to $Mn^{2+}$. This is because $Mn^{3+}$ has a $3d^4$ electronic configuration, while gaining an electron forms $Mn^{2+}$, which possesses a highly stable, exactly half-filled $3d^5$ configuration. Thus, the favourable change from a $d^4$ to a stable $d^5$ configuration is the primary reason for its high reduction potential .