Fluorine shifts from sulfur to metal in difluorosulfane complexes of cyclopentadienyl iron carbonyl: Incompatibility of sulfur-fluorine bonds with iron-iron bonds

The structures and energetics of the cyclopentadienyliron difluorosulfane carbonyls Cp₂Fe₂(SF₂)_m(CO)_n (m = 1, n = 4, 3, 2; m = 2, n = 3, 2) have been investigated using density functional theory. None of the low-energy such structures are found to have short enough Fe-Fe distances to suggest a direct iron-iron bond. This suggests the incompatibility of metal-metal bonds in such structures with sulfur-fluorine bonds. Thus the low energy structures with one SF₂ unit are of the type Cp₂Fe₂(SF)(F)(CO)_n (n = 3, 2) in which either the SF or F unit bridges the iron atoms. For the Cp₂Fe₂(SF₂)₂(CO)_n systems (n = 3, 2), fluorine atoms can migrate from an SF₂ group either to an iron atom or to the other SF₂ group forming an SF₃ ligand. Low-energy structures are found in which an SF₂ group has lost both fluorine atoms in these ways leaving a bare sulfur atom bridging the two iron atoms. However, fluorine migration does not occur in the lowest energy Cp₂Fe₂(SF₂)(CO)₄ structure, which has an intact SF₂ group bridging the iron atoms in two CpFe(CO)₂ units in a structure closely related to SF₄. A related structure with one bridging SF₂ group and one terminal SF₂ group is found for Cp₂Fe₂(SF₂)₂(CO)₃. However, the lowest energy isomer is a structure of the type Cp₂Fe₂(μ-SF)(SF₃)(CO)₃ in which a fluorine atom has migrated from the bridging SF₂ group to the terminal SF₂ group to give a bridging SF ligand and a terminal SF₃ ligand.