Ab initio molecular dynamics and transition path sampling were used to examine the dynamics of the H-2 hydrogenation of ethylene with the catalyst RuHCl (CO)(PR3)(2). The simulations showed H-2 is coordinated very weakly to the metal until ethylene insertion, where it is cleaved into a dihydride intermediate. The dihydride has a shorter lifetime than RRKM theory predicts, due to the localization of kinetic energy into Ru-H vibrational modes after the insertion step.