The experimental spectra are rather similar to each other, see Figure . The bands dominating the optical region, at around 500-550 nm, stem from t2g to π * transitions; these orbitals shift together upon substitution, as it was seen in Figure ; therefore these bands do not show a clear correlation with the EW/ED properties of the Y group. Nevertheless, the bands located in the 300-360 nm region show systematic variations with the 4'-Y substitution, which can be brought to light by examining the first derivative of the spectra (also shown in Figure ). This region consists of an intense and a weak (satellite) peak, the latter at somewhat higher wavelength. Both of these bands exhibit a shift with R + , but with opposite sign, and this is present both in the experimental and calculated spectra, as seen in Figure and Figure , respectively. The calculations reveal that the main peak at ∼ 300 -335 nm is dominated by transitions from π(PyC-C), with a minor Fe-3dxy component, to π * (Fe-Nax)/π * (PyC-C), as an example, see the corresponding natural transition orbitals (NTOs) of the parent molecule, [Fe(terpy)2] 2+ in Figure . The donor orbital is energetically barely affected by the substitution, as no Fe-axial interactions are involved; on the other hand, the energy of the acceptor orbital is more sensitive: it rises (lowers) with ED (EW) substitution, which is in line with the π * (Fe-Nax) character. These energy variations as well as their difference are displayed in Figure ; E(donor): between -7.3 eV and -6.9 eV; E(acceptor): between -3.2 eV and -2.4 eV. Note that these variations can already be appreciated from the average orbital energies in Figure , as the donor and acceptor orbitals belong to the sets denoted as π and π * t2g, respectively, so here even such a single-electron based picture gives reasonable guidelines to the observed the variations. This leads to a red (blue) shift of the main UV peak for EW (ED) substitution. The small peak at higher (>335 nm) wavelengths predominantly arises due to MLCT transition from the Fe-dxy-based orbital to π * (PyC-C) on the two side pyridine rings. While the former (Fe-dxy) is lowered (raised) upon EW (ED) substitution, the latter is less sensitive as there is hardly any contribution on the middle pyridine ring, which is most affected by the 4'-Y substitution. Thus, the small UV peak is blue-(red-)shifted upon EW (ED) substitution. These could have been again predicted from the average orbital energies in Figure ), as the dxy belongs to the t2g, while the π * (PyC-C) belongs to the π * set in the figure. We note that in a previous study on Fe(II) complexes, the authors investigated the effect of substitution by ED and EW groups, in the 4 (γ) position of the Py ring . They showed that these substituents alter the π acceptor character of the ring, and that ED (EW) substitution leads to a blue (red) shift of the affected UV bands in the absorption spectra. (Furthermore, they also assigned the relevant UV bands to LC and MLCT transitions.) This might suggest that the π acceptor character of the ligand is very relevant. The electron configuration for the lowest triplet and quintet state is t2g 5 e * g 1 and t2g 4 e * g 2 in an octahedral complex, here both of these orbitals are split due to the lower symmetry. Most importantly, the antibonding MO based on the d d x 2 -y 2 orbital is populated in the triplet, while both this and the one based on d d z 2 is populated in the quintet state. d dxy , d dxz , d dyz ,