We investigated two types of model, the pyridinic-type model A (FeN 4 C 10 ) and the pyrrolic-type model B (FeN 4 C 12 ), see Figure and SI. In model A, the incorporation of nitrogen in pyridinic, six-membered rings results in the preservation of a fully planar, graphene-like structure for all system sizes. The iron nucleus is therefore coordinated in a square-planar environment; only in model size 2, the iron is moved substantially out of plane (0.6 Å in the HS1 state), leading to a slightly bent structure of the equatorial ligand. In model B, the pyrrolic, fivemembered rings require the incorporation of defects within the graphene plane. We introduced seven-membered rings within the first shell of carbon atoms which results in a possible break of planarity. The planarity changes with the system size: while system sizes 1 to 3 are fully planar, system sizes 4 and 5 are convex models with the iron being moved slightly (approx 0.1 Å) or substantially (0.6 Åfor model size 5, HS1) out of plane compared to a square-planar coordination environment. The model geometries and structural parameters are given in the SI.