Besides local structural changes and ligand field effects, electronic effects such as covalency can play an important role in the electronic structure: if the iron ion sits perfectly centred in a fully planar graphene-like plane, the conjugation between the π system of the plane and the d orbitals of the iron atom will be maximal. The stronger the admixture between the d orbitals and the π system, the more delocalized the d electrons of the iron ion will be, thus exerting a direct influence on the Mössbauer parameters. Viewing conjugation or covalency as the admixture of carbon p z character into the molecular orbitals dominated by iron 3d character defines an easily accessible measure. To better quantify the visually apparent differences in planarity, we furthermore define the "graphene plane angle" as the angle between two axes involving carbon atoms or carbon centroids of two opposite halves of the equatorial ligand, see Figure .