`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.