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 $H_{2}S\left(g\right)$ is passed through this solution?

Answer 1

Copper sulphate reacts with excess $KCN$ to form $\left[Cu\right(CN{)}_4{]}^{2-}$.
Cyanide ion is strong field ligand and forms highly stable complex ion and does not ionize to give $C{u}^{2+}$ ions. When $H_{2}S$ gas is passed, free $C{u}^{2+}$ are not available to form $CuS$ precipitate.