$Ni (28)=3 d^{8} 4 s^{2}$
$Ni ^{2+}=3 d^{8} 4 s^{0}$
Here $XX$ represents electron pairs donated by $Cl ^{-}$ion. $\left[ NiCl _{4}\right]^{2-}$ shows paramagnetic characteristics due to the presence of two unpaired electrons. The $3 d$ orbitals remain undisturbed because $Cl ^{-}$ligand is weak field ligand therefore, $Ni ^{2+}$ ion undergoes $s p^{3}$ hybridisation resulting in tetrahedral structure of the complex.
