What is the coordination entity formed when excess of aqueous KCN is added to an aqueous solution of copper sulphate?

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 H2S(g) is passed through this solution?

Solution:

i.e., 

Thus, the coordination entity formed in the process is $\mathrm{K}_{2}\left[\mathrm{Cu}(\mathrm{CN})_{4}\right] . \mathrm{K}_{2}\left[\mathrm{Cu}(\mathrm{CN})_{4}\right]$ is a very stable complex, which does not ionize to give $\mathrm{Cu}^{2+}$ ions when added to water. Hence, $\mathrm{Cu}^{2+}$ ions are not precipitated when $\mathrm{H}_{2} \mathrm{~S}_{(g)}$ is passed through the solution.

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