The elemental composition of the samples were determined with HPLC-HRMS at Servier Hungária Ltd., using a Kinetex XB-C18 (50 mm×2.1 mm, 2.6 µm) chromatography column and a LC118-TOF high-resolution mass spectrometer. The chromatography revealed a single species in all cases, and the the results confirmed the expected compositions. UV-vis spectra were measured on a Jasco V-750 UV-Visible spectrophotometer. 1 H and C NMR spectra were recorded on a Varian NMR System 400 spectrometer in D 2 O (Aldrich) at 25 • C. Measurements were performed on indirect detection Z probe. Chemical shifts (δ) are expressed in parts per million (ppm). The 1 H chemical shifts are referenced to the residual solvent signal (4.79 ppm). N NMR chemical shifts were also determined from Heteronuclear Multiple Bond Correlation (HMBC) experiments. Fe Mössbauer spectroscopy measurements were performed on a conventional Wissel spectrometer with a sinusoidal velocity profile. A through-flow type liquid He cryostat was used to cool the samples down to low temperatures (77 K with liquid N2 and to 4.2 K with liquid He) and to stabilize the higher (room temperature domain) temperatures. Calibration was performed on a high purity α-iron reference foil with a thickness of 21.1 µm. Isomer shifts are given relative to α-Fe at 294 K with the sign convention used in transmission measurements. The Mössbauer spectra were analyzed with the MossWinn 4.0 program. Cyclic voltammetry Cyclic voltammetry (CV) measurements were per-formed in a standard three-electrode arrangement, using Ag/AgNO3 for reference electrode, and Pt for working and counter electrodes, together with a BioLogic VMP-300 potentiostat. Dry acetonitrile (Sigma-Aldrich) solutions of the PF - 6 salts of the complexes were used, next to a supporting electrolyte of 0.1 M tetrabutylammonium hexafluorophosphate ([ t Bu4N][PF6], Sigma-Aldrich). The stock solutions were dried over 3 Å zeolite beads (Alfa Aesar, 3-5 mm) for at least 24 h. The concentration of the complexes in the solutions was between 1-2 mM. A continuous flow of 5.0 grade Ar gas saturated with acetonitrile was applied to secure a water and oxygen free environment for the samples before and during the measurement. A scan rate of 100 mV/s was applied to record the cyclic voltammogram. The oxidation and reduction potential values were determined as the average of the corresponding cathodic and anodic waves. The reported potential values are the E1 /2 average potentials, referred to the ferrocene/ferrocenium redox couple used as internal reference. Crystallography Single crystal X-ray diffraction was performed using an Agilent Supernova diffractometer equipped with dual microfocus source, kappa goniometer, position sensitive detector and dry nitrogen gas flow cooler. Data were taken at 295 K and 100 K using the Cu (or Mo) source in a hemisphere of the reciprocal space up to 0.8Å resolution. The SHELX software package was used for the solution and refinement of the structure, either via command line scripts or through the Olex2 program .