Molecular geometries were optimized for all complexes with density functional theory (DFT) employing a tight optimization criterion using the ORCA 3.0 program package. In all cases the TZVP basis set and the conductor like screening model (COSMO) were employed, the latter approximates the solvation effects by embedding the molecule in a polarizable medium with a dielectric constant of the solvent, in our case water. Several functionals have been tested, including BP86, B3LYP, B3LYP*, and TPSSh. The theoretical UV-vis spectra were calculated for all corresponding cases with TD-DFT at the Tamm-Dancoff Approximation (TDA); of the functionals tested, B3LYP* was found to provide the best match for the experiments, which is in agreement with a recent study about the validation of DFT functionals for iron(II) complexes. The calculated UV-vis spectra were broadened by a pseudo-Voigt function (with Lorentz and Gauss FWHMs of 900 and 1000 cm -1 , respectively). Mössbauer parameters have also been calculated using the method described in Ref. 66. As the spectra were measured at 77 K, a phenomenological correction was used to approximate the second-order Doppler shift, thus +0.03 mm/s was added to all computed isomer shift values. 1 H NMR and 15 N NMR chemical shifts were calculated at the B3LYP functional with the application the IGLO-II basis set, which was particularly optimized for calculating NMR data. NBO (Natural Bond Orbital Theory) calculations were performed by using the NBO 7.0 software linked to the ORCA package. Moreover, AIM charges were computed within the Multiwfn wavefunction analyser program. Materials 2,2':6',2"-terpyridine and 4'-chloro-2,2':6',2"-terpyridine was purchased from Sigma-Aldrich. The rest of the studied ligands were synthesized at EvoBlocks Ltd., Budapest, in 97-99+% purity. The schemes for the ligand syntheses can be found in the supporting information. The purity and the structure of these ligand materials has been verified by liquid chromatography and 1 H NMR spectroscopy. Methanol and acetonitrile were purchased from Molar Chemicals and FeCl 2 • 4 H 2 O was purchased from Sigma-Aldrich. The synthesis and characterization of several [Fe(4'-Y-terpy)2] 2+ complexes was published in Ref. 22. The complexes discussed here were synthesized using the same general process, with slight variation depending on the solubility of the ligand. In all cases 2.00 molar equivalent of the ligand was dispersed in a suitable solvent. For complexes 1,2,4,8,9,10,11, and 13 H 2 O, while for 3,5,6,7, and 12 methanol was used. Afterwards 0.95 molar equivalent of FeCl 2 • 4 H 2 O was dissolved in the same solvent as the ligand, and was added to the dispersion, which instantly resulted in a dark purple solution. The mixture was stirred at room temperature for 12 hours, after which only the excess ligand remain dispersed. The mixture was filtered on a G4 glass frit funnel to remove the unreacted ligand. The solvent was evaporated at 60 °C in case of H 2 O and 40 °C in case of methanol.