Theoretical study of hydrogen exchange in the reaction of FeC5H6+ with H2

Density functional theory (B3LYP/6-31G*) has been used to study the mechanism of deuterium/hydrogen exchange which is known to occur from mass spectrometric studies of D-2 plus FeC5H6+. The computational results are in accord with the currently accepted mechanism. There is a fast equilibrium between FeC5H6+ and a hydrido-cyclopentadienyl iron complex (HFeC5H5+) which is 3.2 kcal/mol less stable. The FeC5H6+ complex or the HFeC5H5+ complex can coordinate H-2 (14.2 or 10.4 kcal/mol exothermic, respectively). The dihydrogen-hydrogen exchange at the iron center has an activation enthalpy of only 7.1 kcal/mol. Slower exchange of the remaining five hydrogen is enabled by an exo-1,2-hydrogen migration in the cyclopentadiene ring of FeC5H6+ (24.9 kcal/mol) or H-2-FeC5H6+ (23.6 kcal/mol). (C) 2000 Elsevier Science B.V.