Question

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 $H_2S(g)$ is passed through this solution?

Answer 1

$CuSO_{4(aq)}+4KCN_{(aq)}→K_2[Cu(CN)_4]_{(aq)}+K_2SO_4$
i.e,$[Cu(H_2O)_4]^{2+}+4CN^{-}→[Cu(CN)_4]^{2-}+4H_2O$
Thus, the coordination entity formed in the process is $K_2[Cu(CN)_4]$. $K_2[Cu(CN)_4]$ is a very stable complex, which does not ionize to give $Cu^{2+}$ ions when added to water. Hence, $Cu^{2+}$ ions are not precipitated when $H_2S_{(g)}$is passed through the solution.