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The synthesis, characterization, and protonation of [Fe₂(CO)₆{(μ-SCH₂)₂(Et)P═O}] (1) using the moderately strong acid CF₃CO₂H (pK_a^MeCN = 12.7) are reported. Digital simulations of the cyclic voltammetry of 1 in the presence of CF₃CO₂H and DFT calculations have allowed us to obtain a detailed mechanistic picture of the processes underlying the catalytic hydrogen evolution reaction (HER) that 1 can mediate. Moreover, DFT has shed light on the role of the P═O functionality in the whole catalytic cycle of proton reduction. The reductive behavior of 1 features a double electron transfer with potential inversion, which is associated with deep structural rearrangement of the catalyst. The double reduction appears also functional to the intramolecular proton transfer from the P═O group to the diiron core, a crucial process for the H⁺/H^– heterocoupling yielding H₂. The key intermediate for the H₂ formation and …