`[NiCl_(4)]^(2-)` is paramagnetic while `[Ni(CO)_(4)]` is diamgnetic 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 field ligand cannot pair the two unpaired electrons present in 3d-orbitals. This means that 3d-orbitals are not involved in hybridization. Thus, the complex is `sp^(3)` hybridized (tetraheqral) and is paramagnetic in nature.
Outer configuration of Ni atom =

In the complex `[Ni(CO)_(4)]`, the oxidation state of nickel 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 4p-orbitals are involved in hybridization. Thus, the complex is tetrahedral but diamagnetic in nature.
Outer configuration of Ni atom = `3d^(8) 4s^(2)`