Identify the most stable complex species

\([Fe(C_2O_4)_3]^{3–}\)

The correct answer is option 3. \([Fe(C_2O_4)_3]^{3–}\).

Out of the given complex species, the most stable one is [Fe(C₂O₄)₃]³⁻. Here is why:

The stability of the complex depends on the strength of the bond between the central metal ion and the ligands. Stronger bonds lead to more stable complexes.

The order of stability of the ligands follows the trend: C₂O₄²⁻ > CN⁻ > CO > H₂O. This is because the oxalate ion (C₂O₄²⁻) is a bidentate ligand, meaning it can form two coordination bonds with the central metal ion. Cyanide (CN⁻) and carbon monoxide (CO) are also strong σ-bonding ligands, while water (H₂O) is a weak σ-bonding ligand.

In addition to the σ-bonding interaction, the oxalate ion can also participate in π-bonding with the central metal ion due to the presence of delocalized electrons in the π orbitals of the carboxylate groups. This π-bonding further strengthens the bond between Fe³⁺ and C₂O₄²⁻, making [Fe(C₂O₄)₃]³⁻ the most stable complex among the given options.

Here's a table summarizing the stability of the complex species:

Therefore, considering the strength of the bonds and the additional π-bonding interaction, [Fe(C₂O₄)₃]³⁻ emerges as the most stable complex species among the given choices.