Using valence bond theory account for the geometry and magnetic nature of `[NiCl_(4)]^(2-)` ion. `("Atomic number of Ni "= 28)`. 

Explanation

In the given complex ion central metal is nickel with electronic configuration `[Ar]^(18)3d^(8)4s^(2)`
Nickel is in `+2` oxidation state therefore electronic configurations is `Ni^(+2)`

In the given complex, `CN-` is a strong ligand. On the approach of strong ligand the unpaired 3d electron will get pair up. As a result one d - orbital becomes vacant.

The one vacant 3d orbital, one vacant 4s orbital and two vacant 4p orbitals hybridize to give four hybrid orbitals.

Four pairs of electrons one from each `CN-` molecule occupy the geometry is square planar it is diamagnetic complex due to the absence of unpaired electrons.