Kinetic isotope effects for the reactions of Zr (4d25s2,3F) with C2H4 and C2D4 and with C3H6 and C3D6 are measured in a fast flow reactor at 300 K with He/N2 buffer gas at 0.8 Torr. The H2 and D2 elimination products are detected using single photon ionization at 157 nm and time-of-flight mass spectrometry. We find no significant isotope effect for reaction with either ethylene or propylene. These results clearly favor an indirect mechanism involving addition of the metal atom to the CC double bond and subsequent CH insertion and rule out a recently proposed mechanism involving direct CH bond insertion at low collision energy. Density functional theory in its B3LYP/LANL2DZ form finds no barrier to addition of ground state Zr to the double bond of ethylene and confirms the existence of a low-energy path from the metallacyclopropane complex to H2 products. Theory also provides a realistic set of geometries and vibrational frequencies for use in a statistical rate model of the hot metallacyclopropane complex decay. However, the RRKM calculations indicate that a small barrier (0.5−2 kcal/mol) to the approach of Zr and ethylene is necessary to explain our kinetics data and the crossed beam angular distributions of Willis et al. In addition, the barrier to CH insertion must be lowered by 3−6 kcal/mol from the B3LYP/LANL2DZ value.

Identification of the metal-containing products of reactions of the neutral transition metal atom Zr(4d25s2, 3F) with ethylene and propylene is accomplished using one-photon ionization at 157 nm and time-of-flight mass spectrometry. The reactions proceed in a fast flow reactor at 298 K with He/N2 buffer gas at 0.6 Torr. Mass spectra of the products of both Zr + C2H4 and Zr + C3H6 indicate that H2 elimination occurs as the primary reaction step. The efficiency of the Zr + C2H4 reaction shows that there is no barrier larger than about 2 kcal/mol above reactants along the entire reaction path. This corroborates an earlier theoretical prediction by Blomberg and Siegbahn of facile H2 elimination by ground-state Zr. For the secondary reactions ZrC2H2 + C2H4 and ZrC3H4 + C3H6 and for the reactions ZrO + C2H4 and ZrO + C3H6, mass spectra again indicate that H2 elimination occurs. Rate constant measurements using photoionization detection show that the presence of the C2H2 and C3H4 ligands enhances the reaction efficiency over that of the bare Zr atom, while ZrO reacts at essentially the same rate as Zr.