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?

Explanation

On mixing the aqueous solutions of KCN and `CuSO_(4)` the complex formed is potassium tetracyanocuprate(II), since `CN^(-)` ions are strong ligands, the complex is quite stable. It is evident from the stability constant value `(K = 2.0 xx 10^(27))`. It is not cleaved by `H_(2)S` gas when passed through the aqueous solution and no precipitate of CuS is formed.
`CuSO_(4)(aq) + 4KCN(aq) rarr K_(2)[Cu(CN)_(4)] + K_(2)SO_(4)(aq)`
(soluble)