The geometry of ${\left[\mathrm{Ni}(\mathrm{CN}{)}_4\right]}^{2-}\text{ and }{\left[{\mathrm{NiCl}}_4\right]}^{2-}$ ions are :

(NiCl₄) ^2- Cl is weak field ligand which doesn't help in pairing of electron and thus nickel has 2 unpaired electron left due to which it becomes paramagnetic and its configuration is sp³ (tetrahedral) whereas

In (Ni(CN)₄)^2- CN is strong field ligand which helps in pairing of electron thus 2 unpaired electrons of nickel are paired up due to which it becomes diamagnetic leading to configuration dsp²(square planar geometry)