In which of the following order the given complex ions are arranged correctly with respect to their decreasing spin only magnetic moment ?
(i) $\left[\mathrm{FeF}_6\right]^{3-}$
(ii) $\left[\mathrm{Co}\left(\mathrm{NH}_3\right)_3\right]^{3+}$
(iii) $\left[\mathrm{NiCl}_4\right]^{2-}$
(iv) $\left[\mathrm{Cu}\left(\mathrm{NH}_3\right)_4\right]^{2+}$