Three diiron complexes (1-Ph, 2-OH, and 3-OCOFc) as mimics of the diiron subunit of [FeFe]-hydrogenase were electrochemically investigated in 0.1 mol L−1 [NBut4]BF4–acetonitrile (MeCN) under CO and Ar atmosphere. Complex 3-OCOFc was prepared from the reaction of complex 2-OH with ferrocenylacyl chloride (FcOCCl). The complex was fully characterised using a variety of spectroscopic techniques. Its structure was established using X-ray single crystal diffraction analysis. In addition to the well-established ECE (E for electrochemical and C for chemical) mechanism, it was revealed that a further reversible reduction at a potential more negative by ca. 600 mV was observed under CO atmosphere. It was further proposed based on the analysis of electrochemical and infrared spectroscopic data that the second redox was due to a two-electron process of supposedly a tetrairon cluster. This product was formed in situ from the reaction between the dianion generated from the ECE process and its parent complex (1-Ph, 2-OH, and 3-OCOFc) and is supposedly of a core “Fe4(II)”. This reaction occurred only when CO was presented. Under Ar atmosphere, bulk electrolysis led to fully-reduced products, that is, with the iron at the oxidation state of zero, but complex 2-OH was an exception. An overall mechanism to describe the electron transfer and coupled chemical reactions under CO atmosphere was proposed.