Which halogen is known to cause the reaction given below?

All ${Cu}^{2 +}$ halides are known except the iodide. In this case, ${Cu}^{2 +}$ oxidises ${I}^{–}$ to ${I}_{2}$.

$2{Cu}^{2 +} + 4{I}^{-} \to {Cu}_{2}{I}_{2} + {I}_{2}$

However, many copper (I) compounds are unstable in aqueous solution and undergo disproportionation.

$2{Cu}^{+}\overset{}{\to}{Cu}^{2 +} + Cu$