The reaction between ammonia and copper(II) ions

The reaction between ammonia and copper(II) ions

Copper(II) sulphate solution, for example, contains the blue hexaaquacopper(II) ion - [Cu(H₂O)₆]²⁺.

In the first stage of the reaction, the ammonia acts as a Bronsted-Lowry base. With a small amount of ammonia solution, hydrogen ions are pulled off two water molecules in the hexaaqua ion.

This produces a neutral complex - one carrying no charge. If you remove two positively charged hydrogen ions from a 2+ ion, then obviously there isn't going to be any charge left on the ion.

Because of the lack of charge, the neutral complex isn't soluble in water, and so you get a pale blue precipitate.

This precipitate is often written as Cu(OH)₂ and called copper(II) hydroxide. The reaction is reversible because ammonia is only a weak base.

That precipitate dissolves if you add an excess of ammonia solution, giving a deep blue solution.

The ammonia replaces four of the water molecules around the copper to give tetraamminediaquacopper(II) ions. The ammonia uses its lone pair to form a co-ordinate covalent bond (dative covalent bond) with the copper. It is acting as an electron pair donor - a Lewis base.