The Irving–Williams series refers to the relative stabilities of complexes formed by transition metals. In 1953 Harry Irving and Robert Williams observed that the stability of complexes formed by divalent first-row transition metal ions generally increase across the period to a maximum stability at copper: Mn(II) < Fe(II) < Co(II) < Ni(II) < Cu(II) > Zn(II).
Specifically, the Irving–Williams series refers to the exchange of aqua (HO) ligands for any other ligand (L) within a metal complex. In other words, the Irving–Williams series is almost exclusively independent of the nature of the incoming ligand, L.
The main application of the series is to empirically suggest an order of stability within first row transition metal complexes (where the transition metal is in oxidation state II).
Another application of the Irving–Williams series is to use it as a correlation "ruler" in comparing the first stability constant for replacement of water in the aqueous ion by a ligand.