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

The binuclear iron carbonyl derivatives Fe-2(CH2)(CO)(8) and Fe-2(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 C-2v isomers of Fe-2(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 Fe-2(CH2)(CO)(8) isomer with two bridging CO groups and the isomer with all terminal CO groups. The Fe-2(CH2)(CO)(8) system appears to be analogous to the well-documented Co-2(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 Fe-2(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 Fe-2(CF2)(CO)(8) is the C-2v isomer with two symmetrical bridging CO groups as well as a bridging CF2 group. The corresponding Fe-2(CF2)(CO)(8) isomer with all terminal CO groups, also of C-2v symmetry, is also a low-energy structure. In addition to these two low-energy Fe-2(CF2)(CO)(8) structures with a bridging CF2 group, two higher energy structures are found with terminal CF2 groups. The higher energies of similar to 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 Fe-2(CH2)(CO)(8) structures with terminal CH2 groups were found at accessible energies relative to the lowest energy structure. (C) 2012 Elsevier B. V. All rights reserved.