How does ammonia react with a solution of `Cu^(2+)`?

Explanation

`Cu^(2+)` ions react with excess of ammonia to form a deep blue coloured complex.
`Cu^(2+) (aq) + 4 NH_(4)OH(aq) rarr underset(("Deep blue"))underset("Tetramminecopper (II) ion")([Cu(NH_(3))_(4)]^(2+))` + 4H_(2)O`