Binuclear methylene and difluoromethylene iron carbonyls: A density functional theory study

The binuclear iron carbonyl derivatives Fe2(CH2)(CO)8 and Fe2(CF2)(CO)8 are known compounds that have been synthesized and characterized structurally by X-ray crystallography. Their low-lying structures have now been investigated using density functional theory. The C2v isomers of Fe2(CH2)(CO)8 with triply and singly bridged Fe–Fe bonds are found to have closely spaced energies. These observations are consistent with the experimental observation of an equilibrium between the Fe2(CH2)(CO)8 isomer with two bridging CO groups and the isomer with all terminal CO groups. The Fe2(CH2)(CO)8 system appears to be analogous to the well-documented Co2(CO)8 system, where an equilibrium between doubly bridged and unbridged isomers is observed experimentally under ambient conditions. In addition to these two low energy Fe2(CH2)(CO)8 structures, a higher energy structure is found in which one of the CO groups has inserted into an Fe–CH2 bond to give a bridging ketene (CH2CO) ligand. The lowest-lying structure for the corresponding fluorinated derivative Fe2(CF2)(CO)8 is the C2v isomer with two symmetrical bridging CO groups as well as a bridging CF2 group. The corresponding Fe2(CF2)(CO)8 isomer with all terminal CO groups, also of C2v symmetry, is also a low-energy structure. In addition to these two low-energy Fe2(CF2)(CO)8 structures with a bridging CF2 group, two higher energy structures are found with terminal CF2 groups. The higher energies of ∼15 kcal/mol above the lowest energy structure for these terminal CF2 structures suggest that CF2 groups prefer to be bridging rather than terminal ligands in binuclear metal derivatives. No corresponding Fe2(CH2)(CO)8 structures with terminal CH2 groups were found at accessible energies relative to the lowest energy structure.