Density functional calculations have been performed on different conformers and isomers of the propyl group in alkyl cations [L2Zr-Pr](+) (L = Cp, Cp*; Pr = n-propyl), corresponding to two catalysts with different observed rate orders (n) for ethylene polymerization, to explore the single-center, two-state kinetic model for olefin polymerization. For L = Cp (n approximate to 1), the beta-agostic conformer is found to be the most stable structure and also the most reactive with respect to ethylene coordination, which is commensurate with unity rate order. For L = Cp* (n approximate to 1.4), the favored propagation route involves the gamma- and alpha-agostic conformations of the alkyl complex, with coordination taking place to an a-agostic conformation in order to minimize the steric hindrance experienced by the incoming ethylene. The barriers to rearrangement from the alpha- and gamma-agostic conformers to the more stable beta-agostic structure are significantly lower than those of propagation. Moreover, no structure was found to be of lower energy than the beta-agostic conformation, and the latter thus takes the role of the resting state for both catalysts in the present study. Assuming that the single-center, two-state model applies to zirconocene-catalyzed ethylene polymerization, our calculations thus suggest that the corresponding fast and slow, or dormant, states do not originate from equilibria of the alkyl group as investigated here.