Strong field ligands cause pairing of electrons

Electronic configuration of

1) Co³⁺ - [Ar]

Number of unpaired electrons =0

So, [Co(NH₃)₆]³⁺ is diamagnetic.

2) Mn³⁺

As, Mn forms low spin compound with CN lignad, so

[Ar]

Number of unpaired electrons=2

So, [Mn(CN)₆]^3- is paramagnetic

3) Fe²⁺

[Ar] 3d⁴ 4s⁰ and in d orbital there will be pairing because CN is a strong ligand.

Number of unpaired electrons=0

So, [Fe(CN)₆]^4- is diamagnetic

4) Fe³⁺

[Ar]

Number of unpaired electrons =1 because as 3d orbital is half filled and we all know that half filled orbital is more stable than pairing.

So, [Fe(CN)₆]^3- is paramagnetic