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?

Answer 1

Under the given conditions, cupric cyanide is initially generated, which then decomposes to provide cuprous cyanide and cyanogen gas. Cuprous cyanide subsequently dissolves in an excess of potassium cyanide to provide the complex K₃[Cu(CN)₄].

\[\ce{CuSO4 + 2KCN -> Cu(CN)2 + K2SO4 \times 2}\]
\[\ce{2Cu(CN)2 -> Cu2(CN)2 + (CN)2}\]
\[\ce{Cu2(CN)2 + 6KCN -> 2K3[Cu(CN)4]}\]
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\[\ce{2CuSO4 + 10KCN -> 2K3[Cu(CN)4] + 2K2SO4 + (CN)2}\]

So, the coordination entity formed is [Cu(CN)₄]³⁻.

The CN⁻ is a strong field ligand; the complex ion is highly stable and can’t dissociate to give copper ions. Hence, no precipitate of copper sulphide is formed when H₂S_(g) is passed through this solution.