Abstract In order to obtain suitable precursors for iron complexes that model the reactivity of the active sites of nitrogenases, the coordination chemistry of the [Fe(′N2H2S2′)] fragment was systematically investigated(′N2H2S2′2- = 1,2-ethanediamine-N,N′-bis(2-benzenethiolate)(2-)). One-pot reactions of FeCl2 -4H2O with the tetradentate amine-thiolate ligand ′N2H2S2′2- and CO, PR3 or P(OR)3 yielded the complexes [Fe(CO)2(′N2H2S2′)] (1), [Fe(CO)(PR3)(′N2H2S2′)] (R = Et (2), Pr (3), Bu (4)), [Fe(PMe3)2 (′N2H2S2′)] (7), [Fe(dppe)(′N2H2S2′)] (8 , dppe = 1,2- bis(diphenylphosphine)ethane), and [Fe(P(ÖR)3)2(′N2H2S2′)l (R = Me (9), Pr (10)). Mixed phosphane/phosphite complexes [Fe(PMe3)(P(OR)3)(′N2H2S2′)] (R = Me (11), Pr (12)) were synthesized by PMe3/P(OPr)3 exchange of the labile complexes [Fe(PMe3 )2(′N2H2S2′)] (7) and [Fe(P(OPr)3)2(′N2H2S2′)] (10). The [Fe(CO)(PR3)(′N2H2S2′)l complexes 3 and 4 also resulted from 1 and PR3 by photochemical CO substitution. They exhibit characteristic low-frequency v (CO) bands ( ≈1925 cm-1), and their remaining CO ligand proved photolytically inert. Reaction of 3 or 4 with NOBF4 yielded the dinuclear NO complex [Fe(NO)(′N2HS2′)]2 (6 ) which contains two amide functions. Contrary to expectations, [Fe(PR3)2(′N2H2S2′)l complexes could not be obtained with monodentate phosphanes other than PMe3. The corresponding N-methylated ligand ′N2Me2S2,2- yielded only the complexes [Fe(CO)2 (′N2Me2S2′)] (15) and [Fe(′N2Me2S2′)] (16). X-ray structure analyses of complexes 7, 8 -MeOH, 3, [Fe(CO)(dppm)(′N2H2S2 ′)]·0,5 THF (5 ·0,5THF, dppm = bis(diphenylphosphino)methane), and 12·0,5′N2 H2S2 ′-H2 confirm the pseudo-octahedral coordination of the Fe centers by two trans-S and two cis-N donors and two c/s-coligands. The structural data further suggest that the unexpected instability of the [Fe(PR3)2 (′N1H2S2′)] complexes is not caused by steric but by electronic effects. The cyclic voltammograms and the reactivity of the [Fe(L)(L′)(′N2H2S2′)] complexes corroborate this assumption. The oxidation behaviour of the [Fe(L)(L′)(′N2H2S2′)] complexes depends critically on the coligands L and L′. Reaction of 1 with dioxygen leads to oxidative dehydrogenation of the ′N2H2S2′2- ligand and yields the dinuclear Schiff-Base complex [Fe(′gma′ )]2 (′gma′2- = glyoxal-bis(2- mercaptoanil)(2-)). In contrast, oxidation of [Fe(CO)(PR3)(′N2H2S2′)], [Fe(PR3)2 (′N2H2S2′)] and [Fe(P(OR)3)2 (′N2H2S2)] complexes by dioxygen occurs metal-centered and gives the Fe(IV) complexes [Fe(PR3)(′N2S2′)] (R = Me, Pr), [Fe(P(OR)3)(′N2S2′)] (R = Me) and [Fe(′N2S2′)]2 that contain the tetraanionic thiolate-amide ligand ′N2S2′4-.