JEE Main 2026 Apr 8 Shift 2, Chemistry Q21: Colour, Magnetism And Stability Of Co-ordination Compounds

For each complex find the central-metal $d$-electron count, classify the ligand as strong-field (SF) or weak-field (WF) using the spectrochemical series, then count unpaired electrons.

$[{\mathrm{MnBr}}_4{]}^{2-}$: Mn${}^{2+}$ ($d^5$), Br${}^{-}$ is WF, tetrahedral $\Rightarrow$ $5$ unpaired. Paramagnetic.

$[{\mathrm{NiCl}}_4{]}^{2-}$: Ni${}^{2+}$ ($d^8$), Cl${}^{-}$ is WF, tetrahedral $\Rightarrow$ $2$ unpaired. Paramagnetic.

$[{\mathrm{Ni}(\mathrm{CN})}_4{]}^{2-}$: Ni${}^{2+}$ ($d^8$), CN${}^{-}$ is SF, square planar $\Rightarrow$ $0$ unpaired. Diamagnetic.

$[{\mathrm{Ni}(\mathrm{CO})}_4]$: Ni${}^0$ ($d^{10}$), tetrahedral $\Rightarrow$ $0$ unpaired. Diamagnetic.

$[{\mathrm{CoF}}_6{]}^{3-}$: Co${}^{3+}$ ($d^6$), F${}^{-}$ is WF, octahedral high-spin $\Rightarrow$ $4$ unpaired. Paramagnetic.

$[{\mathrm{Fe}(\mathrm{CN})}_6{]}^{4-}$: Fe${}^{2+}$ ($d^6$), CN${}^{-}$ is SF, low-spin $\Rightarrow$ $0$ unpaired. Diamagnetic.

$[{\mathrm{Mn}(\mathrm{CN})}_6{]}^{3-}$: Mn${}^{3+}$ ($d^4$), CN${}^{-}$ SF, low-spin $\Rightarrow$ $2$ unpaired. Paramagnetic.

$[{\mathrm{Ti}(\mathrm{CN})}_6{]}^{3-}$: Ti${}^{3+}$ ($d^1$) $\Rightarrow$ $1$ unpaired. Paramagnetic.

$[{\mathrm{Cu}(\mathrm{H}}_2{\mathrm{O})}_6{]}^{2+}$: Cu${}^{2+}$ ($d^9$) $\Rightarrow$ $1$ unpaired. Paramagnetic.

$[{\mathrm{Co}(\mathrm{C}}_2{\mathrm{O}}_4{)}_3{]}^{3-}$: Co${}^{3+}$ ($d^6$), oxalate is moderate–strong field, low-spin $\Rightarrow$ $0$ unpaired. Diamagnetic.

Paramagnetic count $=6$.