The spin-only magnetic moment value of $\mathrm{M}^{3+}$ ion (in gaseous state) from the pair $\mathrm{Cr}^{3+} / \mathrm{Cr}^{2+}, \mathrm{Mn}^{3+} / \mathrm{Mn}^2$, $\mathrm{Fe}^{3+} / \mathrm{Fe}^{2+}$ and $\mathrm{Co}^{3+} / \mathrm{Co}^{2+}$ that has negative standard electrode potential, is B.M. [Nearest integer]