JEE Advanced 2024 Paper 1, Chemistry Section 3 Q6: Colour, Magnetism And Stability Of Co-ordination Compounds

Determine unpaired electrons in each. NH${}_3$ is a moderate-field ligand and produces low-spin only with strong-field metal centres in 4d/5d; for 3d Co${}^{3+}$ it is high-spin with NH${}_3$ would still typically be low-spin. F${}^{-}$ and Cl${}^{-}$ are weak-field (high-spin). en is a strong-field ligand for 3d M${}^{3+}$ ions, typically low-spin.

$[\mathrm{Mn}(\mathrm{N}{\mathrm{H}}_3{)}_6{]}^{3+}$: Mn${}^{3+}$ is $d^4$. With NH${}_3$ (moderate-field), typically high-spin: 4 unpaired electrons. Paramagnetic.

$[\mathrm{MnC}{\mathrm{l}}_6{]}^{3-}$: Mn${}^{3+}$ $d^4$ with weak-field Cl${}^{-}$: high-spin, 4 unpaired. Paramagnetic.

$[\mathrm{Fe}{\mathrm{F}}_6{]}^{3-}$: Fe${}^{3+}$ $d^5$ with F${}^{-}$: high-spin, 5 unpaired. Paramagnetic.

$[\mathrm{Co}{\mathrm{F}}_6{]}^{3-}$: Co${}^{3+}$ $d^6$ with F${}^{-}$: high-spin, 4 unpaired. Paramagnetic.

$[\mathrm{Fe}(\mathrm{N}{\mathrm{H}}_3{)}_6{]}^{3+}$: Fe${}^{3+}$ $d^5$ with NH${}_3$: high-spin in this case, 5 unpaired. Paramagnetic.

$[\mathrm{Co}(\mathrm{en}{)}_3{]}^{3+}$: Co${}^{3+}$ $d^6$ with the strong-field chelating ligand en: low-spin, $t_{2g}^6{e}_g^0$, all paired. Diamagnetic.

Total diamagnetic species $=1$.