Irving-Williams Series

The Irving-Williams Series refers to the relative stabilities of complexes formed by a metal ion. For high-spin complexes of the divalent ions of first-row transition metals, the stability constant for the formation of a complex follows the order

Mn(II) < Fe(II) < Co(II) < Ni(II) < Cu(II) > Zn(II)

This order was found to hold for a wide variety of ligands.

There are three explanations that are quoted frequently to explain the series.

1. The ionic radius is expected to decrease regularly for Mn2+ to Zn2+. This is the normal periodic trend and would account for the general increase in stability.
2. The Crystal Field Stabilization Energy (CFSE) increases from zero for manganese(II) to a maximum at nickel(II). This makes the complexes increasingly stable. CFSE for zinc(II) is zero.
3. Although the CFSE of copper(II) is less than that of nickel(II), octahedral copper(II) complexes are subject to the Jahn-Teller effect, which affords a complex extra stability.

The actual CFSE values for octahedral systems are 0.4Δ (4 Dq) for iron, 0.8Δ (8 Dq) for cobalt and 1.2Δ (12Dq) for nickel. Δ (10 Dq) is the crystal field splitting energy (the energy gap between the metal-based t2g and eg orbitals). When the stability constants are quantitatively adjusted for these values they follow the trend that is predicted, in the absence of crystal field effects, between manganese and zinc. This was an important factor contributing to the acceptance of crystal field theory, the first theory to successfully account for the thermodynamic, spectroscopic and magnetic properties of complexes of the transition metal ions and precursor to ligand field theory.

However, none of the above three explanations can satisfactorily explain the broad scope of validity of Irving-Williams series (both octahedral and tetrahedral complexes containing different ligands). The recent study of metal-ligand binding in M(II)-thiolate series (M = Mn-Zn) revealed that the interplay between the covalent and electrostatic contributions to metal-ligand binding energies result in Irving-Williams series.

The series is named after H. Irving and Robert Williams from Oxford University who discovered this relationship and subsequently published a paper on it.