The iron-oxido-chlorido complex [(L)FeIV=O(Cl)]+, where L is the tetradentate bispidine 2,4-di(pyridine-2-yl)-3,7-diazabicyclo[3.3.1]nonane-1-one that enforces a cis-disposition of the oxido and chlorido groups has an intermediate spin electronic ground state, leads to selective halogenation with cyclohexane as substrate and is the fastest known nonheme ferryl oxidant. An interesting question is, how much the observed reactivity relates to the FeIV/III=O redox potential, how much to the electrophilicity of the oxido-group and how much it depends on the quintet-triplet energy gap of the ferryl complex. These are fundamental questions for nonheme iron-oxido systems in general, and the proposal aims to find answers to these questions on the basis of experiments and computational work involving the bispidine complexes as well as other systems studied in the research unit. The problem with the redox potential is that it so far is not clear whether any of the published FeIV/III=O redox potentials is accurate and correct, and the various available computational methods have the problem that they need to be validated and calibrated with at least one accurate and reliable experimental data set. Part of the current project aims at solving this problem. The other and most interesting observation is the enormous reactivity of these intermediate-spin systems, and preliminary computational as well as experimental data indicate that this is due to a very small triplet-quintet energy gap. These data need to be confirmed with a set of additional experiments and theoretical studies. This will primarily involve subtle variations of the ligand field, and these are synthetically possible, i.e. up to a dozen examples with the tetradentate bispidine platform and a variation of the donor strengths are available. In terms of data that will be explored experimentally and combined with quantum-chemical analyses, the zero-field splitting (field-Mössbauer and HF-EPR spectroscopy) is the main spectroscopic target. Together with elaborate cryo-stopped-flow kinetics, this might allow us to obtain experimental information on the quintet-triplet energy gap of our ferryl complexes, and this would be an important information for a validation of the computational data.