The high-spin (S = 5/ 2) meridional diastereoisomer of [Fe III(tpena)] 2+(tpena = N,N,N′-tris(2-pyridylmethyl)ethylendiamine-N′-acetate), mer-[Fe(tpena)] 2+, undergoes photolytic CO 2release to produce an iron(II) intermediate of a radical dihydroimidazopyridine ligand (L â). The structure of this unprecedented transient iron(II)(L â) complex is supported by UV-vis and Mössbauer spectroscopies, DFT calculations, as well as the X-ray structural characterization of an μ-oxo iron(III) complex of the oxidized derivative of L â, namely, [Fe III 2O(Cl) 2(L +) 2](ClO 4) 4(MeCN) 2(L += 2-(2-(bis(pyridin-2-ylmethyl)amino)ethyl)-2,3-dihydro-1H-imidazo[1,5-a]pyridin-4-ium). [Fe III 2O(Cl) 2(L +) 2] 4+is obtained only in the absence of O 2. Under aerobic conditions, O 2will intercept the iron(II)(L â) complex to form a putative Fe(III)-alkylperoxide complex which cascades to an iron(II) complex of SBPy3 (SBPy3 = N,N-bis(2-pyridylmethyl)amine-N-ethyl-2-pyridine-2-aldimine). Thus, through different oxidative pathways, the unknown ligand L +or SBPy3 forms by loss of a one-carbon-atom or a two-carbon-atom unit, respectively, from the glycyl arm of tpena. Acceleration of the photodecarboxylation step is achieved by addition of thiocyanate because of transient formation of a more photoreactive NCS -adduct of [Fe(tpena)] 2+. This has allowed for kinetic observation of the reaction of [Fe II(L â)] 2+with O 2which is, unexpectedly, promoted also by light. We propose that this corresponds to the energy needed for the conversion of the ring-closed radical ligand L âto a ring-opened tautomer to allow for O 2insertion between the C and Fe atoms of the iron(II) complex.