`[NiCl_(4)]^(2-)` is paramagnetic while `[Ni(CO)_(4)]` is diamagnetic though both are tetrahedral. Why?

Explanation

In the complex `[NiCl_(4)]^(2-)`, Ni is in `+2` oxidation state and has the configuration `3d^(8)4s^(0)` The `Cl^(-)` ion being a weak ligand cannot pair the two unpaired electrons present in 3d orbitals. This means that 3d orbitals are not involved in hybridisation. The complex is `sp^(3)` hybridised (tetrahedral) and is paramagnetic in nature. In the other complex `[Ni(CO)_(4)]`, the oxidation state of Ni is zero and electronic configuration is `3d^(8)4s^(2)`. In the presence of the ligand CO, the 4s electrons shift to the two half filled 3d orbitals and make all the electrons paired. The valence 4s and 3p orbitals are involved in hybridisation. The complex is tetrahedral but diamagnetic. For more details, consult Section. 10.