The stability constant (log β) for the complex [Fe(CN)₆]⁴⁻ is 36, whereas for [Fe(H₂O)₆]²⁺ it is essentially zero. Which of the following best explains the enormous difference in stability?
(A) CN⁻ is a stronger π-acceptor than H₂O, leading to greater back-bonding and a larger ligand field splitting (Δoct), which stabilizes the low-spin d⁶ configuration.
(B) CN⁻ is a stronger σ-donor than H₂O, which alone accounts for the stability difference.
(C) The high charge density of Fe²⁺ attracts CN⁻ more strongly due to electrostatic interactions.
(D) CN⁻ causes a change in the oxidation state of iron from +2 to +3, which increases stability.
Correct: A
CN⁻ is a strong field ligand that is both a good σ-donor and an excellent π-acceptor. The π-acceptor ability allows significant metal-to-ligand back-bonding, which increases Δoct dramatically. For Fe²⁺ (d⁶), this large Δoct causes pairing of electrons and a low-spin t₂g⁶ configuration, which is highly stable. The large crystal field stabilization energy (CFSE = −2.4Δoct) and additional covalent (back-bonding) stabilization account for the very high formation constant. H₂O is a weak field ligand with small Δoct and negligible back-bonding. Answer: A.