In all single point calculations, the CP(PPP) basis set was used for iron and def2-TZVP for all other atoms and no dispersion correction was applied. Single point calculations for electronic energies used the OPBE functional for 1 and 2 in this work. This functional is chosen based on an in-house calibration study that yielded slightly superior results to the previously employed OLYP density functional. That the most stabilised spin state for model 3 is retained with this setting, was confirmed here (see SI, Table ). Single point calculations for Mössbauer parameters were performed as in an earlier calibration study by Gallenkamp et al. using the B3LYP functional. Calibration of the Mössbauer prediction resulted in trust regions of ± 0.13 mm s -1 for the isomer shift and ± 0.36 mm s -1 for the quadrupole splitting using ORCA 4. With ORCA 5, the calibration study was repeated and resulted in trust regions of again ± 0.13 mm s -1 for the isomer shift and ± 0.22 mm s -1 the quadrupole splitting ‡ .