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

Explanation

First cupric cyanide is formed which decomposes to give cuprous cyanide and cyanogens gas. Cuprous cyanide dissolves in excess of potassium cyanide to form the complex
`K_(3)[Cu(CN)_(4)]`
`CuSO_(4)+2KCN to Cu(CN)_(2) +K_(3) SO_(4)]xx2`
`2Cu(CN)_(2)toCu_(2)(CN)_(2)+(CN)_(2)`
, `Cu_(2) (CN)_(2) +6KCNto 2K_(3)[Cu(CN)_(4)]`
`CuSO_(4)+10KCNto 2K_(3)[Cu(CN)_(4)]+2K_(2)SO_(4)+(CN)_(2)`
Thus the coordination entity formed = [Cu(CN)4] As CN is a strong ligand, the complex ion is highly stable and does not dissociate / ionize to give `Cu^(2+)` Hence, no precipitate with `H_(2)S`.is formed