Metal-benzene and metal-CO bond energies in neutral and ionic C6H6Cr(CO)3 studied by threshold photoelectron-photoion coincidence spectroscopy and density functional theory

Threshold photoelectron-photoion coincidence spectroscopy and density functional theory calculations have been used to investigate the dissociation kinetics of the benzene chromium tricarbonyl ion, BzCr(CO)(3)(+) (Bz = C6H6). The dissociation of the BzCr(CO)(3)(+) ion proceeds by the sequential loss of three CO and benzene ligands. The first and third CO and the benzene loss reactions were associated with metastable precursor ions (lifetimes in the microsecond range). By simulating the resulting asymmetric time-of-flight peak shapes and breakdown diagram, the 0 K appearance energies of the four product ions were determined to be 8.33 +/- 0.05, 8.93 +/- 0.05, 9.97 +/- 0.06, and 11.71 +/- 0.06 eV, respectively. Combined with the ionization energy of BzCr(CO)(3), 7.30 +/- 0.05 eV, the three successive Cr-CO bond energies in the BzCr(CO)(3)(+) were found to alternate, with values of 1.03 +/- 0.05, 0.60 +/- 0.05, and 1.04 +/- 0.05 eV, respectively, and the Bz-Cr bond energy in BzCr(+) is 1.74 +/- 0.05 eV, a trend confirmed by the density functional theory (DFT) calculations. Using the heats of formation of the fully dissociated products, C6H6, Cr+, and CO, the 298 K heats of formation the ionic BzCr(CO)(n)(+) (n = 0-->3) species were determined. By scaling the DFT calculated bond energies for the neutral molecules, the heats of formation of the neutral BzCr(CO)(n) (n = 0-->3) were also obtained.