We have developed over the last ten years a coordination chemistry based on the study of FeCl₂ complexes of small nitrogen-containing tripods derived from tris(2-pyridylmethyl)amine (TPA) ligands. Simple α-substituted ligands were prepared and the corresponding FeCl₂ complexes characterized, at best in solution and in many cases also in the solid state. Depending on their structure as well as on the nature of their ligands, the complexes were found to exhibit a versatile behaviour vs. molecular oxygen, affording new species with generally good yields. This prompted us to try to understand what was hidden beyond the familiar term “oxygen-sensitivity” and to study in more detail the reaction of these simple mononuclear Fe(II) complexes with molecular oxygen. The following report is an overview of several years of our research in this field, and will focus along the following directions: (i) general principles in terms of reactivity and coordination of O₂ to the metal centres, (ii) intramolecular modifications of potentially reactive ligands upon reaction of the complexes with O₂ and (iii) attempts to extend the reactivity to exogenous substrates, i.e. to access biomimetic catalysis. The hypothesis of the inner sphere reduction of O₂ leading to the “superoxide way” as an early step is discussed.