Explain on the basis of valence bond theory that `[Ni(CN)_(4)]^(2-)` ion with square planar structure is diamagnetic and the `[NiCl_(4)]^(2-)` ion with tetrahedral geometry is paramagnetic.

Explanation

Nickel in `[Ni(CN)_(4)]^(2-)` ion is present as `Ni^(2+)` ion.

Thus `Ni^(2+)` ion, with `dsp^(2)-` hybridisation, combines with `4CN^(-)` ions to form `[Ni(CN)_(4)]^(2-)` ion having a square planar structure. The complex ion does not contain any unpaired electron, so it is diamagnetic in nature.
In the formation of `[NiCl_(4)]^(2-)` ion, the unpaired electrons in 3d - orbitas of `Ni^(2+)` ion does not undergo pairing. Here `Ni^(2+)` ion undergoes `sp^(3)` - hybridisation and combines with `4Cl^(-)` ions to form `[NiCl_(4)]^(2-)` ion, having a tetrahedral stucture. The complex ion contains two unpaired electrons and so it is paramagnetic in nature.