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 Cp 2 Fe 2 (SF 2 ) 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 SF 2 unit are of the type Cp 2 Fe 2 (SF)(F)(CO) n ( n = 3, 2) in which either the SF or F unit bridges the iron atoms. For the Cp 2 Fe 2 (SF 2 ) 2 (CO) n systems ( n = 3, 2), fluorine atoms can migrate from an SF 2 group either to an iron atom or to the other SF 2 group forming an SF 3 ligand. Low-energy structures are found in which an SF 2 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 Cp 2 Fe 2 (SF 2 )(CO) 4 structure, which has an intact SF 2 group bridging the iron atoms in two CpFe(CO) 2 units in a structure closely related to SF 4 . A related structure with one bridging SF 2 group and one terminal SF 2 group is found for Cp 2 Fe 2 (SF 2 ) 2 (CO) 3 . However, the lowest energy isomer is a structure of the type Cp 2 Fe 2 (μ-SF)(SF 3 )(CO) 3 in which a fluorine atom has migrated from the bridging SF 2 group to the terminal SF 2 group to give a bridging SF ligand and a terminal SF 3 ligand.