The Role of Haptotropic Shifts in Phosphine Addition to Tricarbonylmanganese Organometallic Complexes: The Indenyl Effect Revisited

Abstract

The addition of PH3 to [(η5-X)Mn(CO)3] was studied by means of molecular orbital calculations performed with the B3LYP HF/DFT hybrid functional. Five reactions, with different π ligands, were compared: X = cyclopentadienyl (Cp = C5H5-), indenyl (Ind = C9H7-), fluorenyl (Flu = C13H9-), cyclohexadienyl (Chd = C6H7-), and 1-hydronaphthalene (Hnaph = C10H9-). In each case, the optimized structures obtained for the η5 complexes are compared with the corresponding experimental X-ray structures and the (η5-X)−Mn bonding is discussed. The results show a η5 coordination geometry in all cases, with a weakening of the polyenyl bonding with the increase of the corresponding π system extension, for Cp, Ind, and Flu. The cyclohexadienyl bonding to the metal proved to be comparable to the indenyl one, and the (η5-Hnaph)−Mn bond is the weaker of the series. The electronic structure of the reaction products, [(η-X)Mn(CO)3(PH3)], was analyzed, and the corresponding optimized geometries were obtained. With the exception of Cp, all the remaining species present a η3 coordination geometry for the polyenyl ligand, X. Folded η3-X were found for Ind, Chd, and Hnaph, and an exocyclic allylic coordination is present in [(η3-Flu)Mn(CO)3(PH3)]. The cyclopentadienyl species has a slipped Cp with a η2 coordination geometry, due to the destabilization associated with the folding of the Cp. The transition states of each of the five reactions were identified, and all present X coordination geometries close to the products. The obtained activation energies indicate the following order for the reaction rates: Cp ≪ Ind ≈ Chd < Flu < Hnaph, which correlates with the (η5-X)−Mn bond strength in the reactants.