Ground state geometries were optimized using the BP86 and B3LYP* functionals. The BP86 results do not show relevant differences in the Fe-N bond lengths while B3LYP* shows a very small but systematic correlation with the substitution. However it is the former that is usually regarded as superior in predicting molecular geometries. The BP86 results agree with previous studies that also found that the Fe-N bond lengths of Py-based ligands are not sensitive to γ-substitution of the Py ring, while they also agree excellently with the experimental crystallographic results (Table ). (There is a slight trend in the r(Fe-Nax) though.) Molecular orbitals, Mössbauer parameters and certain NMR chemical shifts change with substituents, clearly suggesting variations in the electronic structure. Previous studies concluded that ED(EW) groups at the 4 (γ) position make the Py moiety a worse(better) π-acceptor, leading to a decrease(increase) of the ligand field. Similarly, Halcrow and coworkers found that ED(EW) groups at the 4'-position of the Py ring in [Fe(bpp)2] 2+ (bpp=2,6di(pyrazol-1-yl)pyridine) decrease(increase) back bonding. Note that the calculations in these works were done at the ground state geometry only, although magnetization measurements experimentally probe indirectly also the energy of the HS state. However, Ashley et al. argue that it is not only the π-backbonding that is important in this respect. For a first check of the substitution effect, we examine the total Löwdin atomic charges on the atoms of the FeN6 core of the complexes. As it can be seen in Figure , the ED(EW) effects of the substituents have a significant impact on the total charge of the Nax atom, signalling an increased(decreased) electronic density on the nitrogen of the same Py ring. However, the total charge Neqon the side rings is practically unaffected by the 4' substitution on the middle ring. The total charge of the Fe atom also varies with the substituent, although to a much smaller extent. Interestingly, the correlation of the Nax charge is better with the modified Swain-Lupton parameter R + than R, as regression analysis gave R 2 values of 0.97 and 0.87, respectively, so we selected to plot the data over the former one in Figure .