[Fe(CO) 2(‘S 4’)] and [Fe(CO)(‘S 5’)] could be reversibly protonated in two steps by strong acids such as CF 3SO 3H yielding species whose ν(CO) bands are shifted to higher wavenumbers by ∼40 cm −1 per equivalent of acid. The ν(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 FeCO π 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 R 3OBF 4 (RMe, Et), [Fe(CO) 2(‘S 4’)] yielded [Fe(CO) 2(R‘S 4’)]BF 4 (RMe: [ 1a]BF 4, REt: [ 1b]BF 4) and [Fe(CO) 2(R 2‘S 4’)](BF 4) 2 (RMe: [ 3a](BF 4) 2, REt: [ 3b](BF 4) 2). In an analogous way, [Fe(CO)(‘S 5’)] yielded [Fe(CO)(R-‘S 5’)]BF 4 (RMe: [ 2a]BF 4, REt: [ 2b]BF 4) and [Fe(CO)(R 2‘S 5’)](BF 4) 2 (REt: [ 5](BF 4) 2). The ‘asymmetrically’ alkylated [Fe(CO) 2(Me‘S 4’Et)](BF 4) 2, [ 4](BF 4) 2, was obtained by reacting [Fe(CO) 2(‘S 4’)] first with Me 3OBF 4 and subsequently with Et 3OBF 4. Further complexes obtained by alkylation were [Fe(CO)(Bz‘S 5’)]PF 6 ([ 2c]PF 6), containing the benzylated ‘S 5’ ligand, [Fe(CO)(I)(Me 2‘S 4’)]FeI 4 ([ 6]FeI 4) and [Fe(I) 2(Me 2‘S 4’)] ( 7). Except 7, all Fe(II) complexes are diamagnetic containing low-spin Fe(II) centers. Per step of alkylation, the ν(CO) frequencies are raised by ∼40 cm −1 in the case of [Fe(CO) 2(‘S 4’)] and by ∼32 cm −1 in case of [Fe(CO)(‘S 5’)]. These ν(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]BF 4, [ 3a](BF 4) 2, [ 3b](BF 4) 2, [ 5](BF 4) 2, [ 6]FeI 4 and 7. Although the ν(CO) shifts indicate large differences of the electron densities at the iron centers, in a remarkable way the average FeS distances of ∼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 FeS(thiolate) σ-donor bonds into FeS(thioether) σ-donor-π-acceptor bonds upon alkylation. This lowers the electron densities at the iron centers but leaves FeS distances unchanged because weakening of the σ bonds is compensated by the formation of additional π-acceptor bonds. The mono- and dialkylated complexes hydrolyze much more rapidly than the neutral patent 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 N 2 molecule at either FeS or MoS sites of the cofactor in nitrogenases requires previous protonation of the S donors.