What is Irving–Williams series?

The Irving–Williams series is the order of stability of the high-spin complexes of the divalent ions of the first transition series with a given ligand: $\ce{Mn^2+} < \ce{Fe^2+} < \ce{Co^2+} < \ce{Ni^2+} < \ce{Cu^2+} > \ce{Zn^2+}$.