The calculated magnetic moments (spin only value) for species FeCl 4 2 - , Co C 2 O 4 3 3 - and MnO 4 2 -…

(i) ${\left[{\mathrm{FeCl}}_4\right]}^{2-} {\mathrm{Fe}}^{2+} 3{\mathrm{d}}^6\to 4$ unpaired electrons.

$\mathrm{\mu }=\sqrt{\mathrm{n}\left(\mathrm{n}+2\right)}\mathrm{BM}$

$=\sqrt{4\left(4+2\right)}\mathrm{BM}$

$=\sqrt{24} \mathrm{BM}\Rightarrow 4.90 \mathrm{BM}$

(ii) ${\left[\mathrm{Co}{\left({\mathrm{C}}_2{\mathrm{O}}_4\right)}_3\right]}^{-3}$ is a strong field ligand & causes
pairing & hence no. unpaired electron

$\mathrm{\mu }=0$

(iii) ${\left[{\mathrm{MnO}}_4\right]}^{2-} {\mathrm{Mn}}^{+6}$ it has one unpaired electron.

${\mathrm{Mn}}^{+6}\Rightarrow \left[\mathrm{Ar}\right]3 {\mathrm{d}}^1 \mathrm{\mu }=\sqrt{\mathrm{n}\left(\mathrm{n}+2\right)}\mathrm{BM}$

$=\sqrt{1\left(1+2\right)}\mathrm{BM}$

$=\sqrt{3}\mathrm{BM}\Rightarrow 1.73 \mathrm{BM}$