Account for the different magnetic behaviour of `[Ni(CN)_4]^(2-) and [NiCl_4]^(2-)` .

Explanation

In both `[Ni(CN)_4]^(2-) and [NiCl_4]^(2-)`
Ni is present as Ni (II).

Now we know that `CN^(-)` is a strong field ligand so electron will pair up and hence its hybridization is d `sp^2` and complex is diamagnetic. But on the other hand `Cl^(-)` is a weak ligand i.e pairing will not take place and hence in this hybridization is sp3 and it shows paramagnetic character i.e `[NiCl_4]^(2-)` is paramagnetic and `[Ni(CN)_4]^(2-)` is diamagnetic.