The role of haptotropic shifts in phosphine addition to tricarbonylmanganese organometallic complexes: The indenyl effect revisited

The addition of PH3 to [(eta(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 jr 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 eta(5) complexes are compared with the corresponding experimental X-ray structures and the (eta(5)-X)-Mn bonding is discussed. The results show a eta(5) coordination geometry in all cases, with a weakening of the polyenyl bonding with the increase of the corresponding pi system extension, for Cp, Ind, and Flu. The cyclohexadienyl bonding to the metal proved to be comparable to the indenyl one, and the (eta(5)-Hnaph)-Mn bond is the weaker of the series. The electronic structure of the reaction products, [(eta-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 eta(3) coordination geometry for the polyenyl ligand, X. Folded eta(3)-X were found for Ind, Chd, and Hnaph, and an exocyclic allylic coordination is present in [(eta(3)-Flu)Mn(CO)(3)(PH3)]. The cyclopentadienyl species has a slipped Cp with a eta(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 much less than Ind approximate to Chd < Flu < Hnaph, which correlates with the (eta(5)-X)-Mn bond strength in the reactants.