\(N{a_3}[Fe{F_6}]\) \(\to \) \(s{p^3}{d^2}\) hybridisation \(\to \) \(3\) unpaired electrons
\([Cr{({H_2}O)_6}]S{O_4}\) \(\to \) \({d^2}s{p^3}\) hybridisation \(\to \) \(3\) unpaired electrons
\({K_4}[Fe{(CN)_6}]\) \(\to \) \({d^2}s{p^3}\) hybridisation \(\to \) No unpaired electrons
Zero magnetic moment means all the electrons paired.
$(I)$ $\left[\mathrm{Cr}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right] \mathrm{Br}_{2}$
$(II)$ $ \mathrm{Na}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]$
$(III)$ $\mathrm{Na}_{3}\left[\mathrm{Fe}\left(\mathrm{C}_{2} \mathrm{O}_{4}\right)_{3}\right]\left(\Delta_{0}>\mathrm{P}\right)$
$(IV)$ $\left(\mathrm{Et}_{4} \mathrm{N}\right)_{2}\left[\mathrm{CoCl}_{4}\right]$
$(en = NH_2CH_2CH_2NH_2)$