Ammonia forms the complex ${\left[\mathrm{C}\mathrm{u}{\left(\mathrm{N}{\mathrm{H}}_3\right)}_4\right]}^{2+}$ with copper ions in alkaline solution but not in acidic solution. The reason for this is In acidic solution protons are coordinated with ammonia molecules forming $\mathrm{N}{\mathrm{H}}_4^{+}$ ions.
$\mathrm{C}{\mathrm{u}}^{2+}\left(\mathrm{a}\mathrm{q}\right)+4\mathrm{N}{\mathrm{H}}_3\to {\left[\mathrm{C}\mathrm{u}{\left(\mathrm{N}{\mathrm{H}}_3\right)}_4\right]}^{2+}$
${\text{Cu}}^{\text{2+}}\text{+}{\text{4NH}}_{\text{4}}^{\text{+}}\to \left(\text{No}\text{complex}\text{formation}\right)$