Figure 1: Development of the relative spin state energetics (E_Rel) relative to the ground state, and the Mössbauer parameters isomer shift (δ) and quadrupole splitting (ΔE_Q) with an increasing system size of pyridinic (right) and pyrrolic (left) FeNC model complexes. On the x-Axis, the top-view of the models is shown with carbon atoms in grey, nitrogen in blue, iron in orange and hydrogen emitted for clarity. Intermediate spin states (IS) with a total multiplicity 2S+1=3 are shown in yellow, high spin states (HS) with a total multiplicity of 2S+1=5 in red. Spin states with the same occupation of the iron-d-orbitals are given the same number. In the four lower panels, the connected points with solid, large circles correspond to the electronic ground state, the unconnected points with transparent, smaller circles to excited states with relative energies of less than 10 kcal mol^_-1.