Binuclear ethylenedithiolate iron carbonyls: A density functional theory study

The structures and energetics of the binuclear ethylenedithiolate iron carbonyls H2C2S2Fe2(CO)n (n = 8, 7, 6, 5, 4) have been studied using density functional theory. The experimental H2C2S2Fe2(CO)6 structure with the ethylenedithiolate ligand bridging the central Fe?Fe bond through both of its sulfur atoms but with an uncomplexed C??C double bond is predicted to be the lowest energy structure by a margin of -10 kcal/mol. The lowest energy singlet pentacarbonyl H2C2S2Fe2(CO)5 structure can be derived from this hexacarbonyl structure by displacement of one of its CO groups through coordination of the ethylenedithiolate C??C double bond. For the tetracarbonyl H2C2S2Fe2(CO)4 the lowest energy singlet structure has a central distorted bicapped tetrahedral Fe2C2S2 unit shown to be aromatic by a highly negative diatropic nucleus independent chemical shift (NICS) value. Low-energy triplet and quintet structures are also found for the pentacarbonyl and the tetracarbonyl, respectively. The calculated ?E and ?G values for CO dissociation of the carbonyl richer octacarbonyl and heptacarbonyl H2C2S2Fe2(CO)n (n = 8, 7) indicate them to be disfavored relative to formation of the hexacarbonyl H2C2S2Fe2(CO)6 by CO loss.