Density functional theory (DFT) calculations on Fe2S2(CO)(6-2n)(PMe3)(2n) for n = 0, 1, and 2 reveal that the most electron-rich derivatives (n = 2) exist as diferrous disulfides lacking an S-S bond. The thermal interconversion of the Fe-2(II)(S)(2) and Fe-2(I)(S-2) valence isomers is symmetry-forbidden. Related electron-rich diiron complexes [Fe2S2(CN)(2)(CO)(4)](2-) of an uncertain structure are implicated in the biosynthesis of [FeFe]-hydrogenases. Several efforts to synthesize electron-rich derivatives of Fe-2(mu-S-2)(CO)(6) (1) are described. First, salts of iron persulfido cyanides [Fe-2(mu-S-2)(CO)(5)(CN)](-) and [Fe-2(mu-S-2)(CN)-(CO)(4)(PPh3)](-) were prepared by the reactions of NaN(tms)(2) with 1 and Fe-2(mu-S-2)(CO)(5)(PPh3), respectively. Alternative approaches to electron-rich diiron disulfides targeted Fe-2(mu-S-2)(CO)(4)(diphosphine). Whereas the preparation of Fe-2(mu-S-2)(CO)(4)(dppbz) was straightforward, that of Fe-2(mu-S-2)(CO)(4)(dppv) required an indirect route involving the oxidation of Fe-2(mu-SH)(2)(CO)(4)(dppv) (dppbz = C6H4-1,2-(PPh2)(2), dppv = cis-C2H2(PPh2)(2)). DFT calculations indicate that the oxidation of Fe-2(mu-SH)(2)(CO)(4)(dppv) produces singlet diferrous disulfide Fe-2(mu-S)(2)(CO)(4)(dppv), which is sufficiently long-lived as to be trapped by ethylene. The reaction of 1 and dppv mainly afforded Fe-2(mu-SCH=CHPPh2)(mu-SPPh2)(CO)(5), implicating a S-centered reaction.