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

`[Ni(CN)_(4)]^(2-)`
Nickel in the above complex ion is in +2 oxidation state. Formation of `[Ni(CN)_(4)]^(2-)` may be explained through hybridisation as follows :
Configuration of `Ni^(2+)`
Pairing of electrons

followed by `dsp^(2)` hybridisation
It is because of the strong ligand `CN^(-)` that the pairing of electrons takes place. As there are no unpaired electrons, the complex is diamagnetic.
`[NiCl_(4)]^(2-)`
Formation of the above complex may be explained through hybridisation. Ni in the above complex is in +2 oxidation state i.e., as `Ni^(2+)`.
Configuration of `Ni^(2+)`
No pairing of electrons `sp^(3)` hybridisation

As `Cl^(-)` is a weak ligand, pairing of electrons does not take place. `sp^(3)` hybridisation of orbitals takes place giving rise to tetrahedral geometry. As there are two unpaired electrons in the complex, it is paramagnetic.