The reactions of transition metal (M) atoms Zr and Nb with ethylene (C2H4) were studied using the technique of crossed molecular beams. Angular and velocity distributions of MC2H2 products following H-2 elimination were measured at collision energies between 5 and 23 kcal/mol using electron impact and 157 nm photoionization mass spectrometry. Photodepletion studies identify that the atomic reactants are predominantly in their ground electronic states and that the observed MC2H2 products result primarily from reactions of these ground-state atoms. Center-of-mass product angular distributions derived from the data indicate that reactions involve the formation of intermediate complexes having lifetimes longer than their rotational periods. Product translational energy distributions demonstrate that a large fraction of excess available energy is channeled into product internal excitation. Wide-angle nonreactive scattering of metal atom reactants following decay of long-lived MC2H4 association complexes was also observed for both transition metal reactants at collision energies greater than or equal to 9 kcal/mol, with approximately 36% of the initial translational energy converted into C2H4 internal excitation. At collision energies of less than or equal to 6 kcal/mol, nonreactive scattering of Zr from ZrC2H4 decay was found to be negligible, whereas this channel was clearly observed for Nb. RRKM modeling of the competition between decay of MC2H4 complexes back to M + C2H4 and C-H insertion forming HMC2H3 indicates that there exists an adiabatic potential energy barrier for M + C2H4 association in the case of Zr and that the transition state for this process is tighter than for the analogous process in Nb + C2H4. The barrier for Zr + C2H4 association is attributed to the repulsive s(2) ground state configuration of Zr, whereas for Nh the s(1) ground state configuration results in no barrier for association. The absence of decay of ZrC2H4 back to Zr + C2H4 at low collision energies indicates that the barrier for C-H insertion forming HZrC2H3 lies below the barrier for Zr + C2H4 association. This opens up the possibility that direct C-H insertion without initial ZrC2H4 formation may play an important role.