Which one of the following has a square planar geometry? (Co = 27, Ni = 28, Fe = 26, Pt = 78)

Cl- is weak field ligand and so chloride complexes are mostly high spin, but CFSE for transition metals is in the order 3d < 4d < 5d, so the metal belonging to higher series will prefer low spin complex. Among the given complexes, Pt forms low spin complex while others form high spin complex. Square planar geometry of PtCl42− is also supported by its diamagnetic nature.Note: All the complexes of Pd (II) and Pt (II) are square planar.