-2 - In order to obtain further information on how the electronic structure of these complexes is affected by the substitution, it is also interesting to examine the energies of the frontier molecular orbitals, and their variation. The HOMO, HOMO-1, and HOMO-2 orbitals are the 3d atomic Fe orbitals, dxy, dxz, dyz, with a small contribution from mixing with ligand π orbitals. In an octahedral symmetry these would constitute the t2g orbitals, and for simplicity we will often refer to them as such, albeit they are split by the lower (D 2d ) symmetry. The t2g-like 3d orbitals can mix/interact strongly with the unoccupied ligand π * orbitals because of the π-acceptor character of the polypyridine ligands, giving rise to a set of orbitals denoted as π * (t2g), which builds up the LUMO, LUMO+1, LUMO+2 molecular orbitals. The e * g antibonding orbitals stem from the mixing of the d x 2 -y 2 and d z 2 atomic orbitals with the appropriate combinations of the involved N 2p orbitals of the ligands; these are also split by the lower symmetry, and the splitting is as big as 0.5 eV because of the significantly shorter Fe-Nax bond lengths; nevertheless, to follow the variations we will only focus on their average. They are worth to follow, as the gap between the t2g and the e * g reflects the ligand field strength. These (average) orbital energies as well as their variation with the substitution are displayed in Figure . All of these MO en- ergies are affected by the substitution: they decrease with the increase in the EW property of the substituent; however, they follow different slopes. The energy of the e * g varies more slowly than that of the t2g as it is seen from their slopes of m = -0.25 eV and m = -0.32 eV, respectively, which indicates that the ligand field is slightly increasing with the EW substituents. Moreover, it is worth to mention that the axial distortion of the electronic distribution can also be showed while comparing the slope of dxy to dxz and dyz, as the orbitals with z contribution are more impacted by the 4' substitution. Finally, to illustrate the extensive π-3d interactions, we mention two more sets of MOs. One of them is the π d , which stem from occupied ligand π orbitals partly mixing with the t2g-like 3d orbitals, also due to the π-donor character of the polypyridine ligands, constituting the HOMO-7 to HOMO-4 orbitals. The other is further π * orbitals, which again have a slight 3d mixed in, typically making up orbitals between the LUMO+4 and LUMO+7. The variation or their average energies are also shown in Figure . The first oxidation and reduction potentials of the complexes are expected to be related to the energies of the HOMO and the LUMO. We have selected a few complexes, and measured their potentials with cyclic voltammetry; the results are displayed in Table . The measured oxidation and reduction potentials show a relevant monotonic variation with the ED/EW properties, which is seen in Figure , where they are plotted against the R + Swain-Lupton resonance parameter; note that the ordinate is reversed to have a better comparison with the electronic energies. The potential variation is a clear signature of the increase of the energy of the frontier orbitals with electron donation. Interestingly, the difference of the oxidation and reduction potentials stays fairly constant. The oxidation potential is related to the HOMO, which is one of the t2g orbitals, as we saw above. The reduction potential reflects the energy of the LUMO, which is from the ligand π* orbitals mixed with the t2g orbitals. These mean that the ligand π* and the t2g energies are shifted together with the substitution, and their difference stays more or less constant: this is an experimental verification of the affect on the related orbital energies. The potentials, varying in a parallel manner with R + , have slopes of 0.24 -0.26 V, which are comparable to those seen at the calculations, even if they do not agree exactly. The electrode potential changes over the full region span more than 0.6 V, which is comparable to the calculated 0.8 eV variation in the HOMO and the LUMO energies. Finally, from the lack of the systematic variations in the difference of the oxidation and first reduction potentials, we expect no correlation in the optical transitions around the HOMO-LUMO gap. These differences are between 2.2 eV and 2.4 eV, which correspond to excitation wavelengths of 520 -560 nm.