What is the coordination entity formed when excess of aqueous KCN is added to an aqueous solution of copper sulphate? Why is it that no precipitate of copper sulphide is obtained when `H_(2)S` (g) is passed through this solution?

Explanation

First cupric cynide is formed which decomposes to give cuprous cynide and cynogen gas Cuprous cynide dissolves in excees of potassium cyanide to form the complex `K_(3)[Cu(CN)_(4)]`
`CuSO_(4)+2KCN rarr Cu(CN)_(2)+K_(2)SO_(4)]xx2`
`2Cu(CN)_(2)rarr Cu_(2)(CN)_(2)+(CN)_(2)`
`Cu_(2)(CN)_(2)+6KCN rarr 2K_(3)[Cu(CN)_(4)]`
`2CuSO_(4)+10KCN rarr 2K_(3)[Cu(CN)_(4)]+2K_(2)SO_(4)+(CN)_(2)`
Thus, coordination entity formed in the above reaction is `[Cu(CN)_(4)]^(3-)` As `CN^(-)` is a strong ligand the complex ion is highly stable and does not dissociate/ionize to give `Cu^(2+)` ions Hence, no precipitate with `H_(2)S` is formed