Definition and origin
The Irving–Williams series is an empirical trend, established by Harry Irving and Robert J. P. Williams in a 1953 survey of published stability constants, describing how the stability of complexes formed by high-spin divalent first-row (3d) transition-metal ions varies with the identity of the metal. The order is Mn(II) < Fe(II) < Co(II) < Ni(II) < Cu(II) > Zn(II), rising monotonically to a maximum at copper and then dropping at zinc.
Its defining feature is universality: the same ranking is observed across an enormous range of ligands, from simple amines and carboxylates to complex chelators and protein metal sites. In practice this means that, at equal available concentrations, a more competitive metal such as Cu(II) or Zn(II) will bind a given site more tightly than a weakly competitive one such as Mn(II), almost irrespective of what the site is made of.
How it works mechanistically
Two physical effects combine to produce the trend. First, across the period the ionic radius of the divalent ions contracts from Mn(II) to Zn(II), so electrostatic attraction between the metal and its ligands strengthens and complex stability rises. Second, ligand-field stabilization energy adds an extra, metal-specific contribution that peaks near the d8–d9 configurations of Ni(II) and Cu(II); Cu(II) is further stabilized by Jahn–Teller distortion of its d9 shell, which shortens and tightens its equatorial bonds. Together these push stability to a maximum at copper.
Zinc(II), with a filled d10 shell, receives no ligand-field stabilization, so its complexes are less stable than those of copper and it sits just below the peak. The series is therefore a thermodynamic statement about binding affinity, not about how fast metals exchange; kinetics, compartmentalization, and buffered 'free' metal concentrations determine whether the thermodynamically favored metal actually occupies a site in a living cell.
Relevance to microbial metallomics
Because tighter-binding metals higher in the series would otherwise displace the correct cofactor from many enzymes, cells cannot rely on affinity alone to metalate their proteins correctly. This is the root cause of mismetallation: a manganese- or iron-dependent enzyme exposed to even modest free copper or zinc risks capturing the wrong, more competitive metal and losing activity. The Irving–Williams series thus frames much of the logic of cellular metal homeostasis, or metallostasis.
Microbes counter the series with active machinery rather than passive selectivity. Metal-sensing transcriptional regulators tuned to different affinity set-points, such as the copper sensors CsoR and CueR, the zinc sensors Zur and ZntR, and the manganese sensor MntR, keep buffered cytosolic metal concentrations in an inverse order to the series so that weakly competing metals are held relatively abundant and highly competing metals are kept vanishingly low. Metallochaperones and metal-specific transporters then deliver the intended metal to its target, allowing correct metalation despite the underlying thermodynamic bias.
The series also illuminates host–pathogen conflict at infection sites. Nutritional immunity weapons such as the host protein calprotectin sequester Mn(II) and Zn(II) to starve invaders, while phagocytes can instead poison microbes with a toxic surge of copper. Both tactics exploit the same competition captured by the series, and pathogens defend themselves with dedicated metal uptake, efflux, and detoxification systems.
Named examples
Cyanobacterial and other bacterial studies show that the manganese enzyme of photosynthesis and cytosolic Mn/Fe enzymes must be protected from copper and zinc, which sit higher in the series. In Salmonella and Streptococcus, competition described by the series contributes to zinc or copper toxicity when host defenses flood the pathogen with a competitive metal.
The regulator set-points that invert the series are well characterized: MntR (Mn), Fur (Fe), Zur and ZntR (Zn), and CsoR, CueR, and CopY (Cu) collectively sense their metals over affinity ranges that mirror the Irving–Williams order, ensuring that the most competitive ions are the most tightly restricted. Copper-handling chaperones such as CopZ/Atx1-type proteins and copper-transporting ATPases (CopA) illustrate the delivery-and-efflux strategy cells use to keep the most competitive metal out of the wrong sites.
Key points
- The series orders divalent 3d metal complex stability as Mn(II) < Fe(II) < Co(II) < Ni(II) < Cu(II) > Zn(II), peaking at copper.
- It is essentially ligand-independent, so it predicts metal competition for almost any binding site.
- The trend arises from decreasing ionic radius plus ligand-field (and, for Cu, Jahn–Teller) stabilization.
- It explains mismetallation: without active control, competitive metals like Cu(II) and Zn(II) displace correct cofactors.
- Cells counter it with metal sensors, chaperones, and transporters that keep buffered metal levels in inverse order to the series.
- Irving H. & Williams R. J. P., J. Chem. Soc. 1953, 3192–3210 (original paper) — pubs.rsc.org
- Waldron K. J. & Robinson N. J., Nature Reviews Microbiology 2009 — www.nature.com
- Foster A. W., Osman D. & Robinson N. J., J. Biol. Chem. 2014 — www.jbc.org
Frequently asked questions
What is the Irving–Williams series?
It is the empirical stability order of high-spin divalent first-row transition-metal complexes, Mn(II) < Fe(II) < Co(II) < Ni(II) < Cu(II) > Zn(II), that holds for essentially any ligand and predicts which metal binds a given site most tightly.
Why does the Irving–Williams series peak at copper?
Complex stability rises across the row as the ions get smaller and gain ligand-field stabilization energy, which is maximal near Cu(II); copper is further stabilized by Jahn–Teller distortion of its d9 configuration. Zinc(II) has a filled d10 shell with no ligand-field stabilization, so it falls just below the copper maximum.
How does the Irving–Williams series relate to mismetallation and infection?
Because more competitive metals higher in the series would displace correct cofactors, cells must actively regulate metal availability; failure allows mismetallation. Hosts exploit the same competition during infection, using proteins like calprotectin to withhold Mn and Zn or copper flux to poison microbes.