Transition metal atoms and transition metal ions form complexes by combining with ligands. Co2+ ions form complex ion G. Each G ion contains two Co2+ ions, both of which are octahedrally coordinated. Each G ion contains one O2 molecule, which donates one pair of electrons to each Co2+ ion, and one NH2- ion, which donates one pair of electrons to each Co2+ ion. The remaining ligands are NH3 molecules. Cadmium forms complex ion X, [Cd(NH3)4]2+. When a solution containing CN¯ ions is added to an aqueous solution of X, a ligand exchange reaction takes place, forming complex ion Y. Y contains no NH3 ligands and no H₂O ligands. Y is in a much higher concentration in the mixture than X. The oxidation state and coordination number of cadmium do not change in this reaction. Ethanedioate ions, C2O42-, form complexes with transition element ions. The concentration of C2O42- ions can be found by reaction with acidified Cr2O72- ions. C2O42- ions are protonated and form HOOCCOOH molecules which are oxidised by Cr2O72-. The half-equations are shown. Cr2O72- + 14H+ + 6e¯ = 2Cr3+ + 7H2O 2CO2 + 2H+ + 2e¯ ⇌ HOOCCOOH