Though both
[NiCl4]2− and
[Ni(CO)4] are tetrahedral, their magnetic characters are different. This is due to a difference in the nature of ligangds.
Cl− is a weak field ligand and it does not cause the pairing of unpaired 3d electrons. Hence,
[NiCl4]2− is paramgnetic.
In
Ni(CO)4, Ni is in the zero oxidation state i.e., it has a configuration of
3d84s2 But CO is a strong field ligand. Therefore, it causes the pairing of unpaired 3d electrons. Also, it causes the 4s electrons to shift to the 3d orbital, thereby giving rise to
sp3 hybridization. Since no unpaired electrons are present in this case,
[Ni(CO)4] is diamagnetic.