What is the co-ordination entity formed when excess of aqueous KCN is added to an aqueous solution of copper sulphate ? Why is that no precipitate of copper sulphide is obtained when `H_2 S(g)` is passed through this solution ?

Explanation

`K_2 [Cu(CN)_4]` is formed when excess of aqueous KCN is added to an aqueous solution of `CuSO_4` .
`underset("sulphate soluble")underset("Copper")(CuSO_4(aq)) + underset("cyanide")underset("Potassium")(4KCN(aq)) to underset("tetracyanocuprate (II)")underset("Potassium")(K_2 [Cu(CN)_4]) + K_2 SO_4 (aq)`
As `CN^-` ions are strong ligands the complex is very stable. It is not replaced by `S^(2-)` ions when `H_2 S` gas is passed through the solution and thus no precipitate of CuS is obtained.