Which is not stable under ambient condition?

Copper (I) compounds are unstable in aqueous solution and undergo disproportio-nation. $2 \mathrm{Cu}^{+} \rightarrow \mathrm{Cu}^{2+}+\mathrm{Cu}$ The greater stability of $\mathrm{Cu}^{2+}{ }_{(a q .)}$ than $\mathrm{Cu}^{+}{ }_{(a q .)}$ is due to much more $-v e \Delta_{h y d} H$ of $\mathrm{Cu}^{2+}{ }_{(a q .)}$ than $\mathrm{Cu}_{(a q .)}^{+}$, which compensates for the second ionisation enthalpy of $\mathrm{Cu}$.