observations of Halcrow, as a slight decrease can be seen in the (back) donating energy from the iron to the Nax atom while increasing the ED ability of the 4' substituent (see Table third column), while the donation from the nitrogen atom On the other hand, in case of the (normal) donation from the Nax atom to the iron, the energy results are scattered, possibly due to the inductive effect of the para substituted pyridine moiety. Each Mössbauer spectrum shows the signal from a single species, a quadrupole doublet similar to that of the [Fe(terpy)2] 2+ complex, without any additional components. Although the optimized geometries do not show any significant changes upon substitution of the terpyridine ligand, the theoretical Mössbauer parameters, as well as the measured spectra, reveal a systematic change in the axial charge dis- , they show a fair correlation with the nature of the substituent. The quadrupole splitting is known to reflect the axial distortion of the symmetry of the charge distribution around the Fe nucleus (via the interaction of the electric field gradient arising from said charge distribution and the quadrupole moment of the nucleus). The negative sign of the calculated quadrupole splittings indicate an axially compressed charge distribution, which is obviously the case for these complexes, where r(Fe-Nax) is significantly shorter than r(Fe-Neq). The observed variations clearly indicate the increase(decrease) of the partial negative charge on the Naxatoms with electron donating(withdrawing) substituents, similar to what is expected from the calculated Löwdin atomic charges, as seen in Figure . The isomer shifts show an increase(decrease) of the electron density at the Fe nucleus with the ED(EW) substituents; note that this is inversely related to the 3d electron density, which have an indirect (shielding) effect influencing the contraction/expansion of the core s orbitals, thus increasing/decreasing the charge at the nucleus. It has also been reported before that strong π acceptor ligands with large dπ-pπ back-donation cause a decrease in the isomer shift. Actually, this parallels with the tendency observed in the data measured: as it can be deducted from Figure , the complexes of ligands with the stronger EW substituents, which make them stronger π-acceptors, have smaller isomer shift values than those of the ED substituents. The experimental values were taken for most compounds at two different temperatures, and with two different counter ions. The temperature does not affect the observed quadrupole shifts noticeably, as neither the electronic nor the local atomic structure exhibit changes on this temperature scale. A relatively large difference of 0.07 mm/s is observed between the isomer shift data taken at room and liquid nitrogen temperatures, which is due to the difference in the second-order Doppler effect, a well-known relativistic effect arising from the thermal motion of the atoms. The 1 H NMR spectrum of [Fe(terpy)2] 2+ agreed with the literature spectra by Elsbernd and Beattie. The 1 H NMR spectra of the 4'-substituted complexes are qualitatively similar to that of the parent complex and they reveal the variation of certain chemical shifts with the EW/ED properties of the substituents. The terpy ligand exhibits six resonances, while the 4'-substituted derivatives exhibit the five aromatic signals expected. Protons at 3',5'; 3,3"; 4,4"; 5,5"; and 6,6" positions are related by symmetry and possess the same chemical shifts. The same holds for the complexed lig- The chemical shift of the H atoms next to the substituents, δ 3 ,5 , are rather sensitive to the electron withdrawing and donating effects. This may be understood in terms of the deshielding effect of the EW(ED) substituents in the 4' position that will decrease (increase)the electron density around H3' and H5', resulting in larger(smaller) δ 3 ,5 . There are significantly smaller influences of the substituents on the side-ring protons 3,3"; 4,4"; 5,5"; 6,6", which is consistent with the decreasing deshielding effects on the more distant protons. The effects of substituents on the NMR chemical shifts can be analysed using the Swain-Lupton equation, in the following form: