[Fe(CO)2('S4')] and [Fe(CO)('S5')] could be reversibly protonated in two steps by strong acids such as CF3SO3H yielding species whose nu(CO) bands are shifted to higher wavenumbers by approximately 40 cm-1 per equivalent of acid. The nu(CO) shifts are explained by protonation of the thiolate donors leading to a decrease of electron density at the metal centers and consecutive weakening of Fe-CO pi backbonds. While the protonated species could be detected in solution only, analogous isoelectronic complexes which are alkylated at the thiolate donors were isolated and fully characterized spectroscopically and by X-ray structure analyses. Upon reaction with one or two equivalents of oxonium salts R3OBF4 (R = Me, Et), [Fe(CO)2('S4')] yielded [Fe(CO)2(R-'S4')]BF4 (R = Me: [1a]BF4, R = Et: [1b]BF4) and [Fe(CO)2(R2-'S4')](BF4)2 (R = Me: [3a](BF4)2, R = Et: [3b]BF4)2). In an analogous way, [Fe(CO)('S5')] yielded [Fe(CO)(R-'S5')]BF4 (R = Me: [2a]BF4, R = Et: [2b]BF4) and [Fe(CO)(R2-'S5')](BF4)2 (R = Et: [5](BF4)2). The 'asymmetrically' alkylated [Fe(CO)2(Me-'S4'-Et)](BF4)2, [4](BF4)2, was obtained by reacting [Fe(CO)2('S4')] first with Me3OBF4 and subsequently with Et3OBF4. Further complexes obtained by alkylation were [Fe(CO)(Bz-'S5')]PF6 ([2c]PF6), containing the benzylated 'S5' ligand, [Fe(CO)(I)(Me2-'S4')]FeI4 ([6]FeI4) and [Fe(I)2(Me2-'S4')] (7). Except 7, all Fe(II) complexes are diamagnetic containing low-spin Fe(II) centers. Per step of alkylation, the nu(CO) frequencies are raised by approximately 40 cm-1 in the case of [Fe(CO)2('S4')] and by approximately 32 cm-1 in case of [Fe(CO)('S5')]. These nu(CO) shifts are explained in the same way as for the protonated species and corroborate the assumption that protonation takes place at the thiolate donors. X-ray structure determinations were carried out for [1b]BF4, [3a](BF4)2, [3b](BF4)2, [5](BF4)2, [6]FeI4 and 7. Although the nu(CO) shifts indicate large differences of the electron densities at the iron centers, in a remarkable way the average Fe-S distances of approximately 228 pm remain practically invariant in all low-spin Fe(II) complexes regardless of the charge or the degree of alkylation. This is traced back to the transformation of Fe-S(thiolate) sigma-donor bonds into Fe-S(thioether) sigma-donor-pi-acceptor bonds upon alkylation. This lowers the electron densities at the iron centers but leaves Fe-S distances unchanged because weakening of the sigma bonds is compensated by the formation of additional pi-acceptor bonds. The mono- and dialkylated complexes hydrolyze much more rapidly than the neutral parent complexes and allow the facile synthesis of the corresponding sulfur ligands in the free state. The complexes further prove that 3d metals can form stable complexes not only with crown thioethers but also with open chain thioethers. Correlation of states of protonation of iron sulfur ligand complexes with electron densities at the metal center and expected redox potentials allows the hypothesis to be made that reducing the N2 molecule at either Fe-S or Mo-S sites of the cofactor in nitrogenases requires previous protonation of the S donors.
