GAS-PHASE REACTIONS OF 2ND-ROW TRANSITION-METAL ATOMS WITH SMALL HYDROCARBONS - EXPERIMENT AND THEORY

For reactions of gas phase, ground state, neutral transition metal ato ms from the 4d series with alkanes and alkenes, we combine 300 K kinet ics measurements with ab initio electronic structure calculations to i nfer mechanisms in some detail. The theoretical method PCI-80 with zer o-point energy corrections to the bare potential surface apparently pr oduces bond energies, reaction exothermicities, and even saddle point energies accurate to within 2-3 kcal/mol, provided that the correct gr ound state has been located, which is sometimes difficult. The reactio ns fall into two general categories: termolecular stabilization of lon g-lived M(hydrocarbon) complexes and bimolecular elimination of H-2. B y using the ab initio energies and vibrational frequencies in a statis tical unimolecular rate theory (RRKM theory), we can model the lifetim es of M(hydrocarbon) complexes to assess the plausibility of a saturat ed termolecular mechanism at 1 Torr He. Termolecular examples include the reactions of Pd with alkanes to form long-range eta(2) complexes; the reactions of Rh and Pd with alkenes to form pi complexes; and prob ably the reactions of Y, Zr, Nb, Rh, and Pd with cyclopropane to form CH or CC insertion complexes. In other reactions, all of the evidence indicates a bimolecular H-2 elimination mechanism. Rhodium is unique a mong the 4d metal atoms in effecting H-2 elimination from ethane and l arger alkanes. Yttrium, zirconium, and-niobium almost surely insert in CH bonds of ethylene and larger alkenes, ultimately eliminating H-2. We discuss the general requirements on the pattern of atomic electroni c states that permit efficient CH bond activation and H-2 elimination. The good agreement between the observed reaction rates and the PCI-XO calculations lends confidence to future efforts to apply ab initio te chniques to more complicated catalytic systems, including condensed ph ase reactions involving ligated metal centers.