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

Explanation

In `[NiCl_(4)]^(2-)`, Ni is +2 oxidation state with the configuration `3d^(8)4s^(0)` . `Cl^(-)` is weak ligand. It cannot pair up the electrons in 3d orbitals. Hence, it is paramagnetic. In `[Ni(CO)_(4)]`, Ni is in zero oxidation state with the configuration `3d^(8)4s^(2)`. In the presence of `CO` ligand, the 4s electrons shift to 3d to pair up 3d electrons. Thus, there is no unpaired electron present. Hence, it is diamagnetic.