As opposed to complexes with simpler mono-or bidentate ligands, complexes with tridentate ligands seem more promising candidates for ligand engineering. The reason for this is that the behaviour of complexes with the simpler ligands at a spin state transition depends almost exclusively on the symmetric combination of the metal-ligand bond lengths. In contrast, in complexes with tridentate ligands there are more geometric parameters that play a role in determining the course of a spin transition. This claim is supported by evidence from experiments aimed at to elevating the working temperature of such complexes. In terms of practical applications [Fe(terpy)2] 2+ is a very promising complex, since its quintet state is remarkably stable under certain circumstances. Therefore several recent studies investigated this complex, as well as some of its derivatives and structural analogs. As a result of the efforts of several groups attempting to modify the ligand field in this kind of complexes, some tridentate ligands were identified as promising candidates for low-spin and highspin Fe(II) complexes by stabilization of the MLCT and MLCT states, respectively. By employing the tridentate terpyridine as the parent ligand that we subsequently modify by substitution we are making use of the complexity of these systems in order to achieve long quintet lifetimes. We selected the 4' position for substitution, as we expect that changing the electron density on the central ring of terpyridine will have a large effect on the also on the relaxation dynamics of the quintet state. Geometrical constraints of the ligand force the nitrogen atoms of the peripheral rings to an almost equatorial position in the [Fe(terpy)2] 2+ complex (Neq), while the nitrogens of the central pyridine rings are in an axial position (Nax), significantly closer to the iron atom. As the Fe-N bond length is the most relevant geometrical parameter with respect to spin-state switching, varying the electronic structure of the central ring by substitution allows us to modify the potential energy surfaces along this very important dimension.