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

`CuSO_(4)+4KCN to K_(2)[Cu(CN)_(4)]+K_(2)SO_(4)`
(i) Thus , coordination entity formed `=[Cu(CN)_(4)]^(2-)`
(ii) The complex ion is stable and does not dissociate ( ionize ) to give `Cu^(2+)` and `CN^(-)` ions. Hence , no precipitation with `H_(2)S(g)` is formed.