\(2 \mathrm{CuSO}_4+4 \mathrm{KCN} \rightarrow(\mathrm{CN})_2+2 \mathrm{CuCN}+2 \mathrm{~K}_2 \mathrm{SO}_4\)
On combining solution of copper (II) salts and cyanides, an unstable copper (III) cyanide is formed which rapidly decomposes into copper (I) cyanide and cyanogen.