The `E_(M^(2+) // M)^@` for copper is positive (0.34 V). Copper is the only metal in the first series of transition elements showing this behaviour.

Explanation

`E_(M^(2+)//M)^@` depend on enthalpy of atomisation, ionisation energy and hydration energy. Due to high enthalpy of atomisation and low enthalpy of hydration of copper, the ionisation energy needed is not compensated by the energy released. Hence `E^@(Cu^(2+)//Cu)` is positive.