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?

(N/A) The reaction between copper sulphate and excess potassium cyanide is: $CuSO_{4(aq)} + 4KCN_{(aq)} \to K_{2}[Cu(CN)_{4}]_{(aq)} + K_{2}SO_{4(aq)}$.
In ionic form: $[Cu(H_{2}O)_{4}]^{2+} + 4CN^{-} \to [Cu(CN)_{4}]^{2-} + 4H_{2}O$.
The coordination entity formed is the tetracyanocuprate$(II)$ ion,$[Cu(CN)_{4}]^{2-}$,which exists as the salt $K_{2}[Cu(CN)_{4}]$.
$K_{2}[Cu(CN)_{4}]$ is a highly stable complex that does not dissociate to provide free $Cu^{2+}$ ions in the solution.
Since the concentration of free $Cu^{2+}$ ions is extremely low,the ionic product does not exceed the solubility product of $CuS$,and therefore no precipitate of copper sulphide is obtained when $H_{2}S_{(g)}$ is passed through the solution.