Electronic factors for protonation of an organometallic molecule. photoelectron spectroscopy and electron paramagnetic resonance study of [(eta(6)-C6H6)Mo(TRIPOD)](0/+)

We have previously shown that the arene complex (eta(6)-C6H6)Mo(TRIPOD), wh ere TRIPOD = 1,1,1-tris((diphenylphosphino)methyl)ethane, is protonated by exo addition of H+ to the arene ring to give the transient cyclohexadienyl complex [(eta(5)-C6H6)Mo(TRIPOD)](+), which eventually yields the thermodyn amic molybdenum hydride [(eta(6)-C6H6)Mo(TRIPOD)(H)](+). The present study is a combined experimental and theoretical investigation that reveals the f undamental basis for this mechanism. Photoelectron spectroscopy (PES) is us ed to probe the electronic structure of (eta(6)-C6H6)Mo(TRIPOD) and the pro duction of the [(eta(6)-C6H6)Mo(TRIPOD)](+) cation in the gas phase. The in itial ionizations of (eta(6)-C6H6)Mo(TRIPOD) are from energetically closely spaced orbitals of predominantly metal d character ((2)A(1) and E-2 cation states using C-3v symmetry) that are shifted over 2 eV to lower energy wit h respect to the comparable ionizations of (eta(6)-C6H6)Mo(CO)(3). The oxid ized species [(eta(6)-C6H6)Mo(TRIPOD)](+) is also prepared in solution by e lectrochemical means and through the use of chemical oxidants. The electron paramagnetic resonance (EPR) spectrum of this cation shows arene-proton hy perfine coupling that indicates substantial arena character in the highest occupied orbital. The photoelectron and EPR spectra both provide evidence f or Jahn-Teller distortion of the E-2 positive ion states. Electronic struct ure calculations show that this distortion involves opening of one L-Mo-L a ngle, which effectively creates an open coordination site on the metal for the hydride to occupy in the final thermodynamic product. These experimenta l and computational results show that, in terms of their energy, the e symm etry and al symmetry metal-based orbitals are similarly favored for oxidati ve protonation directly at the metal. The e symmetry orbital has a portion of its density on the arene ring, making access to this orbital by proton a pproach to the exo position. of the arene ring possible. For (eta(6)-C6H6)M o(TRIPOD), exo attack at the arene is favored because the TRIPOD ligand shi elds the e symmetry orbital from direct attack at themetal by the solvated proton. Thus, exo attack is not initiated by proton interaction with an are ne-based orbital but is initiated by proton interaction with the arene port ion of the same e symmetry orbital that directs attack at the metal. Calcul ations predict low barriers for both direct attack at the metal and exo att ack at the arene, with attack at the arene favored for longer metal-proton distances.