Copper sulphate dissolves in excess of KCN to give :

When ${\text{CuSO}}_4$ reacts with KCN, an unstable cupric cyanide ($\text{Cu}(\text{CN}{)}_2$) is initially formed, which decomposes into insoluble cuprous cyanide ($\text{CuCN}$) and cyanogen gas. The $\text{CuCN}$ dissolves in excess KCN to form a stable soluble complex, potassium tetracyanocuprate(I) ${\text{K}}_3[\text{Cu}(\text{CN}{)}_4]$, giving the complex ion $[\text{Cu}(\text{CN}{)}_4{]}^{3-}$.