Inspired by nature, biomimetic iron complexes can be used to act as catalysts in selective oxidation reactions. In this process, high-valent oxo species of the iron complexes are passed through. Besides the ligand design for such iron complexes, the structure of the precursor complexes and the coordination chemical behaviour in solution are key to the high-valent iron-oxo species and to investigate their catalytic activity. In this work, tri- and tetradentate ligands with N- and O-donors were synthesised and investigated. For a bis(pyrazolyl)ethanol ligand, a bisfacial coordination of iron with a {N3}-coordination of the ligand was found. The investigated tetradentate ligands (MeC(Py)2Phen, MeC(Py)2PicMe and NQu3) with {N4}- or {N3O}-donor set always leave two cis-positioned coordination sites for labile co-ligands in an octahedral coordination environment. These are particularly important for catalytic oxidation reactions with high-valent iron-oxo intermediates. For all three tetradentate ligands, a [Fe(IV)L=O]2+ species could be detected by mass spectrometry. UV/Vis spectroscopic investigations also show typical iron-oxo bands. For complexes with the ligand MeC(Py)2Phen, the catalytic C-H bond oxidation could also be successfully investigated. The structure of the corresponding precursor complexes was investigated in the solid state and in solution. The main focus was on the co-ligand competition of triflate and acetonitrile. While different species were observed in the solid state, all results in solution suggest that acetonitrile is coordinating when used as a solvent. Furthermore, spin crossover behaviour in solution was observed for complexes with ligands MeC(Py)2PicMe and NQu3 in acetonitrile. A large number of potential precursor complexes with (weakly) coordinating anions could be structurally characterised by single crystal X-ray diffraction. Selected substances were also investigated with magnetometry or Mößbauer spectroscopy. UV/Vis spectroscopy, NMR spectroscopy, magnetometry or Mößbauer spectroscopy were used for coordination chemical investigations in solution. The low-spin complex [Fe(MeC(Py)2Phen)(MeCN)2](OTf)2 was investigated for short-lived excited states by femtosecond X-ray emission spectroscopy and compared with the photophysical properties of a bisfacial iron(II) complex of a bis(pyrazolyl)pyridinylmethane ligand. Both show similar photophysical behaviour to that of the well-studied iron(II) bipyridinyl complex.