A 3Fe4S cluster related to M-cluster of Mo-nitrogenase is reported. [K(THF) 5 ][Fe 3 (μ-bdt) 2 (μ-PPh 2 )(CO) 5 ] (1) is synthesized from photo-assisted structural rearrangement of [K(THF) 2 ][(μ,κ 2 -bdt)Fe 2 (μ-PPh 2 )(CO) 5 ] under visible light irradiation. The molecular structure of 1 consists of a Fe 3 core bearing one dithiolate bridge and the second dithiolate group capping onto the metallic plane. The structural motif of 1 is analogous to that of Mo-participated 3Fe4S unit in M-cluster of Mo-nitrogenase, with the similar Fe-Fe and Fe-S bond distances. Upon protonation in 193 K, a Fe-hydride species (1H) is generated and characterized to possess the bridging hydride group (δ = À12.57 ppm) by 1 H-NMR spectroscopy and DFT calculation. In CH 3 CN solution, complex 1 exhibits a reversible reduction and oxidation process at E red 1=2 = À1.94 V and E ox 1=2 = À0.18 V, respectively, at 273 K. The reduction behavior of 1 in CH 2 Cl 2 solution at 243 K displays a slight modification in the presence of trifluoroacetic acid, revealing a moderate anodic potential shift ($50 mV). The current amplitude of the reduction wave is linearly increased with the increasement of acid added, indicative of the catalytic event.