Which complex changes its geometry from tetrahedral to square planar when Cl− ligands are replaced by CN−?
[NiCl₄]2− (Ni₂+, d8) is tetrahedral with weak field Cl−. With strong field CN−, it becomes [Ni(CN)4]2−, square planar.
Ref: NCERT Class 12 Chemistry > Chapter 5: Coordination Compounds > Topic: Crystal Field Theory - CFT and Crystal Field Splitting and CFSE