Which from following cations forms least stable complex with same ligand?

Step-by-step solution

Complex stability depends on cation charge, ionic radius, and electronic configuration. All given cations ( Co ²⁺ , Fe ²⁺ , Cd ²⁺ , Cu ²⁺ ) share a +2 charge, so differentiation arises from size and crystal field effects. Ionic radii for high-spin, octahedral ions are approximately Cu ²⁺ : 73 pm, Co ²⁺ : 74.5 pm, Fe ²⁺ : 78 pm, Cd ²⁺ : 95 pm. Larger radii correlate with lower charge density and weaker ligand attraction. Electronic configurations yield crystal field stabilization energies: Fe ²⁺ ( d^6 ), Co ²⁺ ( d^7