Ligand substitution. Endpoint of a ligand substitution (or exchange) reaction using an ionic copper solution and concentrated hydrochloric acid (HCl). Copper ions are initially present in a complex with 6 water ligands: [Cu(H2O)6]2+ or hexaaquacopper (II). These ions give the solution a blue colour. When HCl is added, chloride replaces water as a ligand, forming [CuCl4]2- or tetrachlorocuprate (II) complex ions. These ions produce a yellowish solution; however, because the reaction is reversible, the mixture of blue and yellowish complex ions produces the bright green colour seen here.