A [(FeFeII)-Fe-II] dithiolate complex containing acyl and carbonyl ligands was synthesized. The diiron complex reacted with phosphine, cyanide, isocyanide. and CO to give monomeric Fe-II complexes reproducing the first coordination sphere of the active site of [Fe]-hydrogenase (H-2-forming methylene-tetrahydromethanopterin dehydrogenase. Hmd) in various states. All the acyl and carbonyl carbons In the diiron complex Underwent facile isotopic exchange with (CO)-C-13 via monomeric Fe tricarbonyl intermediates.