With the help of valence bond theory expiaiil that tetracyanonickelate (II) ion is square planar or tetrahedral in nature.

Explanation

Nickel has electronic configuration in its ground state is `[Ar]3d^8 4s^2 4p^0`. It loses electrons to from `Ni^(2+)` ion. Now the hybridisation and formation of complex `[Ni(CN)_4]^(2-)` is given as below :

The two unpaired d- electrons are paired up. The one vacant 3d-orbital get hybridised with one 4s- orbital and two 4p-orbitals, resulting in to `dsp^2` hybridisation. Now these four vacant hybrid orbitals are occupied by four pairs of electrons from four `CN^(-)` ions. Thus fonned complex `[Ni(CN)_4]^(2-)` is square planar. This complex has no unpaired electrons. Hence, it is diamagnetic in nature.