JEE Main 2025 Jan 22 Shift 2, Chemistry Q10: Colour, Magnetism And Stability Of Co-ordination Compounds

CFSE increases with (i) larger crystal-field splitting $\Delta$, which grows with higher metal oxidation state and stronger field ligands, and (ii) octahedral geometry over tetrahedral (since ${\Delta }_t\approx \frac{4}{9}{\Delta }_o$).

Comparing the four complexes:

$[Co(N{H}_3{)}_4{]}^{2+}$: $C{o}^{2+}$, tetrahedral, weak field, smallest CFSE.

$[Co(N{H}_3{)}_6{]}^{2+}$: $C{o}^{2+}$, octahedral with $N{H}_3$, larger than tetrahedral but charge is only $+2$.

$[Co(N{H}_3{)}_6{]}^{3+}$: $C{o}^{3+}$, octahedral with $N{H}_3$. Higher charge increases ${\Delta }_o$ substantially.

$[Co(en{)}_3{]}^{3+}$: $C{o}^{3+}$, octahedral with en, which is a stronger field ligand than $N{H}_3$ in the spectrochemical series, giving the largest ${\Delta }_o$ and hence highest CFSE.

$\therefore$ Order: $[Co(N{H}_3{)}_4{]}^{2+}<[Co(N{H}_3{)}_6{]}^{2+}<[Co(N{H}_3{)}_6{]}^{3+}<[Co(en{)}_3{]}^{3+}$.