Which of the following complex will show largest splitting of d-orbitals?

🧠 Largest Splitting = Strongest-Field Ligand × Highest Charge

All four complexes have Fe(III) with 6 ligands (octahedral). So the only variable is the ligand.

🗺️ Spectrochemical Series

$\mathrm{F^- < C_2O_4^{2-} < H_2O < NH_3 < CN^-}$

| Complex | Ligand | $\Delta_o$ | |---|---|---| | $[\mathrm{Fe(C_2O_4)_3}]^{3-}$ | $\mathrm{C_2O_4^{2-}}$ | mild | | $[\mathrm{FeF_6}]^{3-}$ | F⁻ | weak | | $[\mathrm{Fe(CN)_6}]^{3-}$ | CN⁻ | largest ✓ | | $[\mathrm{Fe(NH_3)_6}]^{3+}$ | NH₃ | strong (but less than CN⁻) |

CN⁻ is at the top of the spectrochemical series → produces the largest $\Delta_o$.

⚡ CN⁻ Dominates Among Common Ligands

CN⁻ outranks all common ligands in field strength because of its strong $\sigma$-donation and strong $\pi$-acceptor capability. Both effects together push $\Delta_o$ to the maximum.

⚠️ $[\mathrm{Fe(NH_3)_6}]^{3+}$ Is Unstable

In aqueous conditions, Fe³⁺ + NH₃ doesn't actually give $[\mathrm{Fe(NH_3)_6}]^{3+}$ — it gives $\mathrm{Fe(OH)_3}$ precipitate due to NH₃ acting as a base. The complex exists in some non-aqueous conditions but is not the canonical reference. Despite this, the field-strength ordering still holds.

$\boxed{\text{Answer: (3) } [\mathrm{Fe(CN)_6}]^{3-}}$