Why [Fe(CN)₆]^4- is diamagnetic where as [Fe(H₂O)₆]⁺² is highly paramagnetic ?

In both the given complexes the metal ion is same that is Fe(II) which is a d⁶  and the ligands are different. 

The different magnetic behaviour of these two complexes can be explained with the help of either crystal field theory or valence bond theory.

 From spectro chemical series it is evident that CN ^– is a very strong field ligand while H₂O is comparatively weak ligand.

 So, according to crystals field theory the Δ₀ value or 10 dq value is very high in case of CN ^– ligand and  actually exceeds the pairing energy resulting in the formation of a low spin complex. 

The crystal field splitting of d-electrons of Fe(II) in [Fe(CN)₆]^4- complex  is shown below,

Since in this configuration there is no unpaired electron, the complex [Fe(CN)₆]^4- is diamagnetic.

But in [Fe(H₂O)₆]⁺²  the Δ₀ value is less than pairing energy resulting in the formation of high spin complex having electronic configuration t_2g4 e_g2. The crystal field splitting of d-electrons of Fe(II) in [Fe(H₂O)₆]⁺²  complex  is shown below, 

From the above  d-orbital splitting it has been found that Fe(II) ion contains four unpaired electrons.So the complex is strongly paramagnetic.

According to valence bond theory in [Fe(CN)₆]^{4 –} complex Fe⁺²  involves d²sp3 hybridization that is inner orbital complex having no unpaired electron and hence it is diamagnetic. 

But in [Fe(H₂O)₆]⁺² the hybridization is sp3d² that is outer orbital complex having four unpaired electron and hence it is highly paramagnetic in nature.

Summary

• What is crystal field theory ?
• What is valence bond theory ?
• Why [Fe(CN)6]4- is diamagnetic where as [Fe(H2O)6]+2  is highly paramagnetic ?

Click here: valence bond theory ( V.B.T )