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

The addition of PH 3 to [(η 5 -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 = C 5 H 5 - ), indenyl (Ind = C 9 H 7 - ), fluorenyl (Flu = C 13 H 9 - ), cyclohexadienyl (Chd = C 6 H 7 - ), and 1-hydronaphthalene (Hnaph = C 10 H 9 - ). In each case, the optimized structures obtained for the η 5 complexes are compared with the corresponding experimental X-ray structures and the (η 5 -X)−Mn bonding is discussed. The results show a η 5 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 (η 5 -Hnaph)−Mn bond is the weaker of the series. The electronic structure of the reaction products, [(η-X)Mn(CO) 3 (PH 3 )], was analyzed, and the corresponding optimized geometries were obtained. With the exception of Cp, all the remaining species present a η 3 coordination geometry for the polyenyl ligand, X. Folded η 3 -X were found for Ind, Chd, and Hnaph, and an exocyclic allylic coordination is present in [(η 3 -Flu)Mn(CO) 3 (PH 3 )]. The cyclopentadienyl species has a slipped Cp with a η 2 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 (η 5 -X)−Mn bond strength in the reactants.