(a) Cl- is a weak field ligand as compared to CN-, which do not allow the pairing of electrons therefore in complex [NiCl4]2- there are two unpaired electron, due to which the complex is paramagnetic.
(b) In tetrahedral complex the splitting of d- orbital is smaller or due to less CFSE it is never energetically favourable to pair up the electrons therefore tetrahedral complexes are rare.
