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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?

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i. When excess of aqueous KCN is added to an aqueous solution of copper sulphate, the coordination entity formed is [Cu(CN)4]2 -.
[Cu(H2O)4]2+ + 4CN -  →   [Cu(CN)4]2 -  + 4H2O
ii. [Cu(CN)4]2- is a coordination compound and hence, retains its identity in the aqueous solution. It does not dissociate to give free Cu2+ ions. Hence, no precipitate of copper sulphide is obtained when H2S(g) is passed through this solution

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[ C u ( C N ) 4 ] 2-

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CuSO4(aq) + 4KCN(aq) → K2Cu (CN)4](aq) + K2SO4

i.e. [Cu (H2O)4]2+ + 4CN- → [Cu(CN)4]2- + 4H2O

Thus, the coordination entity formed in the process is K2 [Cu (CN)4]. K2 [Cu (CN)4] is a very stable complex which does not ionize to give Cu2+ ions when added to water. Hence Cu2+ ions are not precipitated when H2S(g) is passed through the solution.

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