Among the following complexes, the total number of diamagnetic species is_____
\(\left[\mathrm{Mn}\left(\mathrm{NH}_3\right)_6\right]^{3+},\left[\mathrm{MnCl}_6\right]^{3-},\left[\mathrm{FeF}_6\right]^{3-},\left[\mathrm{CoF}_6\right]^{3-},\)\(\left[\mathrm{Fe}\left(\mathrm{NH}_3\right)_6\right]^{3+}\), and \(\left[\mathrm{Co}(\mathrm{en})_3\right]^{3+}\)
[Given, atomic number: \(\mathrm{Mn}=25, \mathrm{Fe}=26, \mathrm{Co}=27\);
\(\text { en } \left.=\mathrm{H}_2 \mathrm{NCH}_2 \mathrm{CH}_2 \mathrm{NH}_2\right]\)

\(\mathrm{Mn}^{3+} \Rightarrow[\mathrm{Ar}] 3 \mathrm{~d}^4\)
\(\mathrm{d}^4\) configuration in \(\mathrm{t}_{2 \mathrm{~g}}\) and \(\mathrm{eg}_{\mathrm{g}}\) orbitals will always have unpaired electrons irrespective of SFL and WFL.
\(\mathrm{Fe}^{3+} \Rightarrow[\mathrm{Ar}] 3 \mathrm{~d}^5\)
\(\mathrm{d}^5\) configuration will also have unpaired electron irrespective of SFL and WFL.
\(\mathrm{Co}^{3+} \Rightarrow[\mathrm{Ar}] 3 \mathrm{~d}^6\)
\(\mathrm{d}^6 \Rightarrow\) it can be both paramagnetic or diamagnetic based on field of ligands.
In case of \(\mathrm{F}^{-} \Rightarrow\) weak field ligand, configuration will be \(\mathrm{t}_{2 \mathrm{~g}}^4 \mathrm{e}_{\mathrm{g}}^2\) hence it is paramagnetic but in case of
en \(\Rightarrow\) strong filed ligand, configuration will be \(\mathrm{t}_{2 \mathrm{~g}}^6 \mathrm{e}_{\mathrm{g}}^0\) hence it will be diamagnetic.
So,
\(\left[\mathrm{Mn}\left(\mathrm{NH}_3\right) 6\right]^{3+}\) : Paramagnetic
\(\left[\mathrm{MnCl}_6\right]^{3-}\) : Paramagnetic
\(\left[\mathrm{FeF}_6\right]^{3-}\) : Paramagnetic
\(\left[\mathrm{CoF}_6\right]^{3-}\) : Paramagnetic
\(\left[\mathrm{Fe}\left(\mathrm{NH}_3\right)_6\right]^{3+}\) : Paramagnetic
\(\left[\mathrm{Co}(\mathrm{en})_3\right]^{3+}\) : Diamagnetic
Only 1 complex is diamagnetic.