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 cyanide is formed which decomposes to give cuprous cyanide and cyanogen gas. Cuprous cyanide dissolves in excess of potassium cyanide to form the complex, `K_(3)[Cu(CN)_(4)]`
`CuSO_(4)+2 KCNrarrCu(CN)_(2)+K_(2)SO_(4)xx2`
`2Cu(CN)_(2)rarrCu_(2)(CN)_(2)+(CN)_(2)`
`(Cu_(2)(CN)_(2)+6KCNrarr2K_(3)[Cu(CN)_(4)])/(2CuSO_(4)+10KCNrarr2K_(3)[Cu(CN)_(4)]+2K_(2)SO_(4)+(CN)_(2))`
Thus, coordination entity formed `=[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.