This work is focused on the identification and investigation of the catalytically relevant key iron species in a photocatalytic proton reduction system described by Beller and co‐workers. The system is driven by visible light and consists of the low‐cost [Fe3(CO)12] as catalyst precursor, electron‐poor phosphines P(R)3 as co‐catalysts, and a standard iridium‐based photosensitizer dissolved in a mixture of THF, water, and the sacrificial reagent triethylamine. The catalytic reaction system was investigated by operando continuous‐flow FTIR spectroscopy coupled with H2 gas volumetry, as well as by X‐ray absorption spectroscopy, NMR spectroscopy, DFT calculations, and cyclic voltammetry. Several iron carbonyl species were identified, all of which emerge throughout the catalytic process. Depending on the applied P(R)3, the iron carbonyl species were finally converted into [Fe2(CO)6(μ‐CO){μ‐P(R)2}]−. This involves a P−C cleavage reaction. The requirements of P(R)3 and the necessary reaction conditions are specified. [Fe2(CO)6(μ‐CO){μ‐P(R)2}]− represents a self‐assembling, sulfur‐free [FeFe]‐hydrogenase active‐site mimic and shows good catalytic activity if the substituent R is electron poor. Deactivation mechanisms have also been investigated, for example, the decomposition of the photosensitizer or processes observed in the case of excessive amounts of P(R)3. [Fe2(CO)6(μ‐CO){μ‐P(R)2}]− has potential for future applications.