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

The reaction may be explained in the following steps :

Thus, coordination entity formed in the above reaction is : `[Cu(CN)_(4)]^(3-)`
The complex ion is highly stable and does not dissociate to give `Cu^(2+)` ions. Hence, no precipitate with `H_(2)S` is obtained.