Stereochemistry of alkylthioiron carbonyls and isovalent cyclopentadienylcobalt and benzeneiron derivatives

Density functional calculations on the Fe2(CO)6(μ-SR)2, Co2(η5-C5H5)2(μ-SR)2, and Fe2(η6-C6H6)2(μ-SR)2 systems (R = CH3, CF3) predict butterfly structures with a non-planar central M2S2 unit containing one M–M bond for the “body” of the butterfly and four M–S bonds at the edges of the “wings” of the butterfly. These butterfly structures are found to be of three types. In every case the lowest energy structures are those with the two RS groups oriented in opposite directions (the ud isomers). The other two possible stereoisomers, namely the uu and dd isomers with equivalent RS groups, are found at significantly higher relative energies. Structures with planar Fe2S2 units with 180° S–Fe–Fe–S dihedral angles are found for the Fe2(CO)6(μ-SR)2 derivatives but always at significantly higher energies than the butterfly structures, ranging from 21 to 29 kcal/mol above the global minima. The nature of the RS substituents has no significant effect on the relative stability of the isomers.