The Irving–Williams series refers to high-spin, octahedral, divalent metal ion of the first transition series. It places the stabilities of complexes in the order
:Mn < Fe < Co < Ni < Cu > Zn
This order was found to hold for a wide variety of ligands. There are three strands to the explanation of the series.
#The ionic radius is expected to decrease regularly for Mn to Zn. This would be the normal periodic trend and would account for the general increase in stability.
#The crystal field stabilisation energy (CFSE) increases from zero for manganese(II) to a maximum at nickel(II). This makes the complexes increasingly stable. CFSE returns to zero for zinc(II).
#Although the CFSE for copper(II) is less than for nickel(II), octahedral copper(II) complexes are subject to the Jahn–Teller effect which results in a complex having extra stability.
Another example of the effect of ionic radius the steady increase in stability of complexes with a given ligand along the series of trivalent lanthanide ions, an effect of the well-known lanthanide contraction.