JEE Main · 2025 · 24 Jan 2025 · Shift 2 · Coordination Compounds

Adding ethane-1,2-diamine (en) to aqueous $\mathrm{NiC}{\mathrm{l}}_2$ progressively replaces the aqua ligands of $[\mathrm{Ni}({\mathrm{H}}_2\mathrm{O}{)}_6{]}^{2+}$ with the bidentate chelate one step at a time, and each substitution shifts the crystal field splitting and hence the observed color:

$[\mathrm{Ni}({\mathrm{H}}_2\mathrm{O}{)}_6{]}^{2+}$ — green

$[\mathrm{Ni}({\mathrm{H}}_2\mathrm{O}{)}_4(\mathrm{en}){]}^{2+}$ — pale blue (one en coordinated)

$[\mathrm{Ni}({\mathrm{H}}_2\mathrm{O}{)}_2(\mathrm{en}{)}_2{]}^{2+}$ — blue (two en coordinated)

$[\mathrm{Ni}(\mathrm{en}{)}_3{]}^{2+}$ — violet (all three chelate rings closed, strongest ligand field)

The sequence of colors observed as en is added progressively is Green → Pale Blue → Blue → Violet.