Question

Compare the magnetic behaviour of the complex entities [Fe(CN)₆]⁴⁻ and [FeF₆]³⁻.

Answer 1

• The magnetic behaviours of the complexes [Fe(CN)₆]⁴⁻ and [FeF₆]³⁻ differ significantly due to the nature of the ligands and their effect on the electronic configuration of the central metal ion.
• In [Fe(CN)₆]⁴⁻, iron is in the +2 oxidation state with a 3d⁶ configuration. The ligand cyanide (CN⁻) is a strong field ligand that causes significant pairing of electrons within the lower-energy t_2g orbitals.
• As a result, all six electrons pair up, and the complex becomes diamagnetic with no unpaired electrons. In contrast, [FeF₆]³⁻ has iron in the +3 oxidation state, with a 3d⁵ configuration.
• The ligand fluoride (F⁻) is a weak field ligand, which does not cause electron pairing, so the five d-electrons remain unpaired across both t_2g and e_g orbitals.
• This leads to a high-spin complex that is strongly paramagnetic due to the presence of five unpaired electrons. Therefore, [Fe(CN)₆]⁴⁻ is diamagnetic, while [FeF₆]³⁻ is paramagnetic.