If 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) $\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6}\right]^{3+}$
(iii) $\left[\mathrm{NiCl}_{4}\right]^{2-}$
(iv) $\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\right]^{2+}$
Correct Option: , 4
$\left[\mathrm{FeF}_{6}\right]^{3-} \mathrm{Fe}^{3+} 3 \mathrm{~d}^{5} \rightarrow 5$-unpaired electrons as $\mathrm{F}^{-}$is weal field legend
$\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6}\right]^{3+} \mathrm{Co}^{3+} 3 \mathrm{~d}^{6} \rightarrow$ No-unpaired electron as $\mathrm{NH}_{3}$ is strong field light and causes pairing
$=[\mathrm{NiCl} 4]^{2-} \mathrm{Ni}^{2+} 3 \mathrm{~d}^{8} \rightarrow 2$-unpaired electrons
$\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\right]^{2+} \mathrm{Cu}^{2+} 3 \mathrm{~d}^{9} \rightarrow 1$-unpaired electrons