Question

How would you account for the following?

[Fe(CN)₆]³⁻ is weakly paramagnetic, while [Fe(CN)₆]⁴⁻ is diamagnetic.

Answer 1

The difference in magnetic behaviour between [Fe(CN)₆]³⁻ and [Fe(CN)₆]⁴⁻ is due to their differing oxidation states and resulting electron configurations. In [Fe(CN)₆]³⁻, iron is in the +3 oxidation state (Fe³⁺), which has a 3d⁵ configuration. Although cyanide (CN⁻) is a strong field ligand and tends to pair electrons, in the case of Fe³⁺, one unpaired electron still remains even after pairing. This results in the complex being weakly paramagnetic. In contrast, in [Fe(CN)₆]⁴⁻, iron is in the +2 oxidation state (Fe²⁺) with a 3d⁶ configuration. Due to the strong field nature of CN⁻, all six electrons pair up within the lower energy t₂g orbitals. As a result, there are no unpaired electrons in the complex, making [Fe(CN)₆]⁴⁻ diamagnetic. Therefore, the magnetic behaviour directly correlates with the quantity of unpaired electrons in the d-orbitals of the central metal ion.