The rate of exchange of $\mathrm{OH}_{2}$ present in the coordination sphere by $^{18}\mathrm{OH}_{2}$ of, (i) $\left[\mathrm{Cu}\left(\mathrm{OH}_{2}\right)_{6}\right]^{2+}$ (ii) $\left[\mathrm{Mn}\left(\mathrm{OH}_{2}\right)_{6}\right]^{2+}$ (iii) $\left[\mathrm{Fe}\left(\mathrm{OH}_{2}\right)\right]_{6}^{2+}$ (iv) $\left[\mathrm{Ni}\left(\mathrm{OH}_{2}\right)\right]_{6}^{2+}$ follows an order