For models with different degrees of planarity, it can be reasonably expected that the covalency, i.e. the admixture of ligand and metal character in the molecular orbitals, changes significantly. Nevertheless, for the present models no empirical rule that would connect covalency with Mössbauer parameters was found. Similar to discussions in previous work , this reinforces the need to construct broad ranges of FeNC models with theoretical methods, and to allow flexibility in the coordination sphere of the iron ion.