For reactions of gas phase, ground state, neutral transition metal atoms from the 4d series with alkanes and alkenes, we combine 300 K kinetics measurements with ab initio electronic structure calculations to infer mechanisms in some detail. The theoretical method PCI-80 with zero-point energy corrections to the bare potential surface apparently produces bond energies, reaction exothermicities, and even saddle point energies accurate to within 2-3 kcal/mol, provided that the correct ground state has been located, which is sometimes difficult. The reactions fall into two general categories : termolecular stabilization of long-lived M(hydrocarbon) complexes and bimolecular elimination of H-2. By using the ab initio energies and vibrational frequencies in a statistical unimolecular rate theory (RRKM theory), we can model the lifetimes of M(hydrocarbon) complexes to assess the plausibility of a saturated 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 probably 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 among the 4d metal atoms in effecting H-2 elimination from ethane and larger 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 electronic 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 techniques to more complicated catalytic systems, including condensed phase reactions involving ligated metal centers.