JEE Main · 2023 · 13 Apr 2023 · Shift 2 · Coordination Compounds

Magnetic attraction is proportional to the number of unpaired electrons. Analyzing each complex:
$[\mathrm{Zn}({\mathrm{H}}_2\mathrm{O}{)}_6{]}^{2+}$: Zn²⁺ is d¹⁰, 0 unpaired electrons.
$[\mathrm{Co}({\mathrm{H}}_2\mathrm{O}{)}_6{]}^{2+}$: Co²⁺ is d⁷, weak-field ligand → high-spin → ${\mathrm{t}}_{2g}^5{\mathrm{e}}_g^2$, 3 unpaired electrons.
$[\mathrm{Co}(\mathrm{en}{)}_3{]}^{3+}$: Co³⁺ is d⁶, strong-field ligand → low-spin → ${\mathrm{t}}_{2g}^6{\mathrm{e}}_g^0$, 0 unpaired electrons.
$[\mathrm{Ni}({\mathrm{H}}_2\mathrm{O}{)}_6{]}^{2+}$: Ni²⁺ is d⁸ → ${\mathrm{t}}_{2g}^6{\mathrm{e}}_g^2$, 2 unpaired electrons.
The complex with the maximum unpaired electrons (3) is $[\mathrm{Co}({\mathrm{H}}_2\mathrm{O}{)}_6{]}^{2+}$.