For a given ligand the stability of the complexes of the metallic ions having the same charge decreases with the increase in the size of the central metal ion. The sequence of the given ions in is $\mathrm{Mn}^{2+} < \mathrm{Fe}^{2+} < \mathrm{Co}^{2+} < \mathrm{Ni}^{2+} < \mathrm{Cu}^{2+} < \mathrm{Zn}^{2+}$. This sequence is known as the Irving-William order of stability of the complexes of $\mathrm{M}^{2+}$ ions.
.