In the separation of Cu²⁺ and Cd²⁺ in 2nd group qualitiative analysis of cations, tetram-mine copper (II) sulphate and tetrammine cadmium (II) sulphate react with KCN to form the corresponding cyano complexes. Which one of the following pairs of the complexes and their relative stability enables the separation of Cu²⁺ and Cd²⁺ ?

K₃[Cu(CN)₄] is more stable than K₂[Cd(CN)₄] because the ionization of the copper in K₃[Cu(CN)₄] is +1 which is lower than the ionization of Cd in K₂[Cd(CN)₄], which is +2. So, the solution has a high concentration of cadmium ions because cadmium complexes ionizes easily and when H₂S passes on the solution, cadmium ion separated as CdS.