The hapticity of octafluorocyclooctatetraene in its first-row mononuclear transition metal carbonyl complexes: effect of perfluorination

The iron tricarbonyl complex of octafluorocyclooctatetraene was synthesized by Hughes and co-workers and shown by X-ray crystallography to have a trihapto-monohapto structure (eta(3,1)-C8F8)Fe(CO)(3) in contrast to the tetrahapto structure (eta(4)-C8H8)Fe(CO)(3) formed by the non-fluorinated cyclooctatetraene. This difference has stimulated a comprehensive density functional theoretical study of the octafluorocyclooctatetraene metal carbonyl complexes (C8F8)M(CO) (n) (n = 4, 3, 2, 1 for M = Ti, V, Cr, Mn, and Fe; n = 3, 2, 1 for M = Co, Ni) for comparison with their hydrogen analogues (C8H8)M(CO) (n) . In most such systems, the substitution of fluorine for hydrogen leads to relatively small changes in the preferred structures. However, for the iron carbonyl derivatives (C8X8)Fe(CO)(3) (X = H, F), the difference observed experimentally has been confirmed by theory with (eta(3,1)-C8F8)Fe(CO)(3) and (eta(4)-C8H8)Fe(CO)(3) being the lowest energy structures by 4 and 14 kcal/mol, respectively. The ligand exchange reactions C8H8 + (C8F8)M(CO) (n) -> C8F8 + (C8H8)M(CO) (n) are predicted to be exothermic for almost all of the systems considered, with the (eta(3,1)-C8X8)Fe(CO)(3) system being the main exception. This suggests that the C8F8 ligand generally bonds more weakly to transition metals than the C8H8 ligand in accord with the electron-withdrawing effect of the ligand fluorine atoms.