Order of CFSE \((\Delta)\) \(\underset{\text { (Tetrahedral) }}{\Delta_{t}} < \underset{\text { (Octahedral) }}{\Delta_{0}} < \underset{\text { (Square planar) }}{\Delta_{s p}}\) Absorb energy \(=\Delta\) \(\left[\mathrm{NiCl}_{4}\right]^{2-}\) and \(\left[\mathrm{Ni}(\mathrm{CO})_{4}\right]\) have tetrahedral geometry. So, they have low absorption energy. \(\mathrm{Cl}^{-}\)is a weaker ligand than \(\mathrm{CO}\) hence, \(\left[\mathrm{NiCl}_{4}\right]^{2-}\) exhibits lower energy absorption band.
