Data mining of iron(II) and iron(III) bond-valence parameters, and their relevance for macromolecular crystallography

Using all available metal-containing organic compound structures in the Cambridge Structural Database, a novel data-driven method to derive bond-valence R 0 parameters was developed. While confirming almost all reference literature values, two distinct populations of FeII—N and FeIII—N bonds are observed, which are interpreted as low-spin and high-spin states of the coordinating iron. Based on the R 0 parameters derived here, guidelines for the modeling of iron–ligand distances in macromolecular structures are suggested.