You are an engineer studying materials for computer hard-drive fabrication and need to predict the magnetic properties of the iron complexes you are investigating. Compare the magnetic properties of

(a) [Fe(OH₂)₆]²⁺;

(b) [Fe(CN)₆]^4–.

ANTICIPATE Because H₂O is a weak-field ligand, you should expect the maximum number of unpaired electrons; hence, it is likely to be high spin and thus paramagnetic. The CN₂ ion is a strong-field ion and so its complexes are likely to be low spin (and possibly diamagnetic).

PLAN To predict the magnetic properties of a complex, you need to know both the number of d-electrons on the central atom or ion and the position of the ligands in the spectrochemical series. Decide from the spectrochemical series whether the ligands are weak field or strong field. Then judge whether the complex is high spin or low spin.