[Fe(CN)₆]^4– is diamagnetic while [FeF₆]^4– is strongly paramagnetic. Why?

[Fe(CN)₆]⁴⁻ In the above coordination complex, iron exists in the +II oxidation state. Fe²⁺. Electronic configuration of Fe²⁺is 4s⁰ 3d⁶.
As CN⁻ is a strong field ligand, it causes the pairing of the unpaired 3d electrons.
Since there are six ligands around the central metal ion, the most feasible hybridization is d^₂sp³. d²sp³ hybridized orbitals of Fe²⁺ . 6 electron pairs from CN⁻ ions occupy the six hybrid d²sp³orbitals. Then,



Since CN^- is strong field ligand it cause pairing. hence it is diamagnetic.

In this complex, the oxidation state of Fe is +3. eletronic configuration is 4s⁰ 3d⁵. 
There are 6 F⁻ ions. Thus, it will undergo d²sp³ or sp³d² hybridization. As F⁻ is a weak field ligand, it does not cause the pairing of the electrons in the 3d orbital. Hence, the most feasible hybridization is sp³d^2.



Since [FeF₆]^4– have unpaired electrons. Hence it is strongly paramagnetic.