The geometry of Ni(CN)42− and NiCl42− 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) whereasIn (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)