JEE Main · 2026 · 05 Apr 2026 · Shift 2 · Coordination Compounds

Oxalate in $[\mathrm{Fe}({\mathrm{C}}_2{\mathrm{O}}_4{)}_3{]}^{3-}$ is a bidentate chelating ligand, and the chelate effect makes it the most stable of the three iron(III) complexes listed, so statement (A) is correct. Ethylenediamine is also bidentate, so $[\mathrm{Cu}(\mathrm{en}{)}_2{]}^{2+}$ is more stable than $[\mathrm{Cu}(\mathrm{N}{\mathrm{H}}_3{)}_4{]}^{2+}$, making statement (B) incorrect. In ${\mathrm{K}}_4[\mathrm{Fe}(\mathrm{CN}{)}_6]$ iron is $+2$ (${\mathrm{d}}^6$) and the strong-field $\mathrm{C}{\mathrm{N}}^{-}$ pairs the electrons to ${\mathrm{t}}_{2g}^6{\mathrm{e}}_{\mathrm{g}}^0$, freeing two inner $3d$ orbitals for ${\mathrm{d}}^2\mathrm{s}{\mathrm{p}}^3$ hybridisation, so statement (C) is correct. $\mathrm{N}{\mathrm{O}}_2^{-}$ is ambidentate (bonding through N or O), so $[\mathrm{Fe}(\mathrm{N}{\mathrm{O}}_2{)}_3\mathrm{C}{\mathrm{l}}_3{]}^{3-}$ shows linkage isomerism and statement (D) is correct; statement (E) is wrong because both $\mathrm{N}{\mathrm{O}}_2^{-}$ and $\mathrm{SC}{\mathrm{N}}^{-}$ are ambidentate. The correct statements are A, C and D only.