JEE Main · 2026 · 04 Apr 2026 · Shift 2 · Coordination Compounds

Nickel(II) is a $3d^8$ ion. In an octahedral field the $d^8$ set is $t_{2g}^6{e}_g^2$ for any ligand strength, so both $[\mathrm{Ni}(\mathrm{en}{)}_3{]}^{2+}$ and $[\mathrm{Ni}(\mathrm{N}{\mathrm{H}}_3{)}_6{]}^{2+}$ carry two unpaired electrons. In the tetrahedral $[\mathrm{NiC}{\mathrm{l}}_4{]}^{2-}$ the configuration is $e^4{t}_2^4$, again two unpaired electrons. The frequency of the d--d absorption rises with the crystal-field splitting energy. The splitting grows with ligand-field strength ($\mathrm{en}>\mathrm{N}{\mathrm{H}}_3>\mathrm{C}{\mathrm{l}}^{-}$) and an octahedral field splits far more than a tetrahedral one (${\Delta }_o\approx \frac{9}{4}{\Delta }_t$), so the splitting increases as $[\mathrm{NiC}{\mathrm{l}}_4{]}^{2-}<[\mathrm{Ni}(\mathrm{N}{\mathrm{H}}_3{)}_6{]}^{2+}<[\mathrm{Ni}(\mathrm{en}{)}_3{]}^{2+}$. Each complex has two unpaired electrons, and the absorption frequency order is $(A)>(C)>(B)$.