Five series of [2Fe-2S] complexes, Fe(2)S(2)Cl(2)(-)(x)(CN)(x), Fe(2)S(2)(SEt)(2)(-)(x)Cl(x), Fe(2)S(2)(SEt)(2)(-)(x)(CN)(x), Fe(2)S(2)Cl(2)(-)(x)(OAc)(x) (OAc = acetate), and Fe(2)S(2)(SEt)(2)(-)(x)(OPr)(x) (OPr = propionate) (x = 0-2), were produced by collision-induced dissociation of the corresponding [4Fe-4S] complexes, and their electronic structures were studied by photoelectron spectroscopy. All the [2Fe-2S] complexes contain a Fe(2)S(2) core similar to that in reduced [2Fe] ferredoxins but with different coordination geometries. For the first three series, which only involve tricoordinated Fe sites, a linear relationship between the measured binding energies and the substitution number (x) was observed, revealing the independent ligand contributions to the total electron binding energies. The effect of the ligand increases in the order SEt --> Cl --> CN, conforming to their electron-withdrawing ability in the same order. The carboxylate ligands in the Fe(2)S(2)Cl(2)(-)(x)(OAc)(x) and Fe(2)S(2)(SEt)(2)(-)(x)(OPr)(x) complexes were observed to act as bidentate ligands, giving rise to tetracoordinated iron sites. This is different from their monodentate coordination behavior in the [4Fe-4S] cubane complexes, reflecting the high reactivity of the unsatisfied three-coordinate iron site in the [2Fe-2S] complexes. The [2Fe-2S] complexes with tetracoordinated iron sites exhibit lower electron binding energies, that is, higher reductive activity than the all tricoordinate planar clusters. The electronic structures of all the [2Fe-2S] complexes were shown to conform to the "inverted energy level scheme".