Electronic Factors for Protonation of an Organometallic Molecule. Photoelectron Spectroscopy and Electron Paramagnetic Resonance Study of [(η 6 -C 6 H 6 )Mo(TRIPOD)] 0/+ Victor S. Asirvatham, 1 Nadine E. Gruhn, 2 Dennis L. Lichtenberger, 2 and Michael T. Ashby* ,1 Department of Chemistry and Biochemistry, The University of Oklahoma, 620 Parrington Oval, Room 208, Norman, Oklahoma 73019-0370, and Center for Gas-Phase Electron Spectroscopy, Department of Chemistry, University of Arizona, Tucson, Arizona 85721-0041 Received August 23, 1999 We have previously shown that the arene complex (η 6 -C 6 H 6 )Mo(TRIPOD), where 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 [(η 5 -C 6 H 7 )Mo(TRIPOD)] + , which eventually yields the thermodynamic molybdenum hydride [(η 6 -C 6 H 6 )Mo(TRIPOD)(H)] + . The present study is a combined experimental and theoretical investigation that reveals the fundamental basis for this mechanism. Photoelectron spectroscopy (PES) is used to probe the electronic structure of (η 6 -C 6 H 6 )Mo(TRIPOD) and the production of the [(η 6 -C 6 H 6 )Mo- (TRIPOD)] + cation in the gas phase. The initial ionizations of (η 6 -C 6 H 6 )Mo(TRIPOD) are from energetically closely spaced orbitals of predominantly metal d character ( 2 A 1 and 2 E cation states using C 3v symmetry) that are shifted over 2 eV to lower energy with respect to the comparable ionizations of (η 6 -C 6 H 6 )Mo(CO) 3 . The oxidized species [(η 6 -C 6 H 6 )Mo- (TRIPOD)] + is also prepared in solution by electrochemical means and through the use of chemical oxidants. The electron paramagnetic resonance (EPR) spectrum of this cation shows arene-proton hyperfine coupling that indicates substantial arene character in the highest occupied orbital. The photoelectron and EPR spectra both provide evidence for Jahn-Teller distortion of the 2 E positive ion states. Electronic structure calculations show that this distortion involves opening of one L-Mo-L angle, which effectively creates an open coordination site on the metal for the hydride to occupy in the final thermodynamic product. These experimental and computational results show that, in terms of their energy, the e symmetry and a 1 symmetry metal-based orbitals are similarly favored for oxidative 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 approach to the exo position of the arene ring possible. For (η 6 -C 6 H 6 )Mo(TRIPOD), exo attack at the arene is favored because the TRIPOD ligand shields the e symmetry orbital from direct attack at the metal by the solvated proton. Thus, exo attack is not initiated by proton interaction with an arene-based orbital but is initiated by proton interaction with the arene portion of the same e symmetry orbital that directs attack at the metal. Calculations predict low barriers for both direct attack at the metal and exo attack at the arene, with attack at the arene favored for longer metal- proton distances. Introduction We have previously reported deuterium tracer studies and kinetic measurements that provided evidence for a mechanism for protonation of (η 6 -C 6 H 6 )Mo(TRIPOD) (1) to give the metal-hydride complex [(η 6 -C 6 H 6 )Mo- (TRIPOD)(H)] + (1‚(H + )), which involves exo protonation of the arene ligand to give the cyclohexadienyl transient [(η 5 -C 6 H 7 )Mo(TRIPOD)] + (1‚(H + ) q ) followed by endo proton transfer to the metal (text figure, right): 3 The question naturally arises, why does the proton initially attack the arene ligand to give [(η 5 -C 6 H 7 )Mo- (TRIPOD)] + rather than attack the metal directly to give [(η 6 -C 6 H 6 )Mo(TRIPOD)(H)] + ? Protonation of the mol- (1) The University of Oklahoma. (2) University of Arizona. (3) Kowalski, A. S.; Ashby, M. T. J. Am. Chem. Soc. 1995, 117, 12639. 2215 Organometallics 2000, 19, 2215-2227 10.1021/om990673u CCC: $19.00 © 2000 American Chemical Society Publication on Web 05/04/2000