Explain, Why `[Ni(CO)_4]` is diamagnetic whereas `[NiCl_4]^(2-)` is paramagnetic ?

Explanation

In case of `[Ni(CO)_4]`, electronic configuration of Ni is `4s^2 3d^8` and in case of `[NiCl_4]^(2-)` is `3d^8`.
In case of `[Ni(CO)_4]` Ni is ixisating as Ni(0) But in `[NiCl_4]^(2-)` Ni is existing as Ni (II)
Also CO is strong.ligand hence electron pairing takes place. But on the other hand in case of `Cl^(-)` as it is weak ligand.no electron pairing takes place.
Due to electron pairing in case of `[Ni(CO)_4]` it is diamagnetic . But `[NiCl_4]^(2-)` is paramagnetic due to presence of unpaired electrons.