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

Explanation

Paramagnetic nature of `[NiCl_(4)]^(2-)` ion has been explained in question no. 12 (in this section). In `Ni(CO)_(4)`, `3d^(8)4s^(2)`. In presence of `CO` ligand, 4s electrons are transferred to 3d - orbitals and hence the configuration of `Ni` becomes `3d^(10)4s^(0)`. Then `sp^(3)`- hybridisation occurs involving 4s and 4 p - orbitals. Finally, Ni - atom combines with four `CO` molecules to form `Ni(CO)_(4)` having a tetrahedral structure. The complex contains no unpaired electrons and hence diamagnetic in nature.