| We have studied the reactions of ground-state transition metal atoms with alkenes. Chemical kinetics data are collected using a fast flow reactor at 300 K with He/N2 buffer gas at 0.5--1.1 Torr. Detection of metal atoms and metal-containing products is accomplished using photoionization mass spectrometry at 157 nm. The experimental results are combined with density functional theory calculations using statistical rate theory to explore the details of the bimolecular reaction mechanisms.; We find that Zr (4d25s2, 3F) reacts with ethylene and propylene to form exclusively H2 elimination products. The room temperature reaction efficiencies are 7% and 18%, respectively. There is no significant isotope effect for reaction with either ethylene or propylene. In contrast, the analogous Y (4d15s2, 2D) reactions produce both H2 elimination products and Y(alkene) stabilized complexes. The reactions are 1% and 16% efficient with ethylene and propylene, respectively. We find a small normal isotope effect for the reaction with ethylene but no significant isotope effect for the reaction with propylene.; We use density functional theory in its B3LYP, mPW1PW91, and B1LYP forms with a large basis set to completely characterize stationary points for the reaction M + C2H4 → MC2H 2 + H2, where M denotes either Y or Zr. For Zr + ethylene, low-lying triplet and singlet reaction paths are identified, but the experimental data is more consistent with primary CH insertion followed by the stepwise elimination of H2 over a small exit barrier on the singlet potential energy surface. For Y + ethylene, with only the doublet electronic surface important, insertion intermediates undergo the concerted elimination of H2 via a low-lying multi-center transition state with no subsequent exit barrier to products.; Statistical rate modeling indicates that B3LYP places key transition state energies too high by 6--9 kcal/mol, while the mPW1PW91 density functional gives much more realistic energies. Results indicate that CH bond insertion is remarkably facile, and that the rate-determining step involves the initial formation of the metallacyclopropane complexes. However, theory does not find any substantial barrier along the adiabatic entrance channels, suggesting that nonadiabatic effects play an Important role in controlling the overall reaction efficiency. |
