What is the coordination entity formed when excess of aqueous KCN is added to aqueous solution of `CuSO_(4)` ? Why is it that no precipitate of copper sulphide is obtained when `H_(2)S(g)` is passed through this solution ?

Explanation

Aqueous solution of copper sulphate contains the complex entity `[Cu(H_(2)O)_(4)]^(2+)`. It reacts with excess of `KCN` solution to form a new coordination entity, cuprocyanide ion, complex) and does not ionise in solution to form `Cu^(2+)` ions.
So, on passing `H_(2)S, CuS` is not formed due to non - availability of `Cu^(2+)` ions in solution.
`2[Cu(H_(2)O)_(4)]^(2+)+underset("(excess)")(10CN^(-))rarr underset("(highly stable)")underset("tetracyanocuprate (I) ion")(2[Cu(CN)_(4)]^(3-)+(CN)_(2))+8H_(2)O`