The reaction of ethylene with Cp(2)Zr(+)-Et has been studied with nonlocal density functional theory (DFT). Comparison of four different chain orientations in Cp(2)Zr(+)-Bu reveals the high stability of a beta-agostic resting state as compared to delta- (+30.9 kJ/mol), gamma-(+26.9 kJ/mol), and alpha-agostic (+46.8 kJ/mol) structures. Thus the beta-agostic Cp(2)Zr(+)-Et was chosen as the model for the resting state. A beta-agostic pi-complex is formed in an exothermic reaction (Delta H = -37.1 kJ/mol). Before insertion can take place, rotation around Zr-C-alpha has to take place in order to form an alpha-agostic intermediate. The electronic barrier for this process amounts to 14.0 kJ/mol and is compensated by a positive entropy of activation (Delta S double dagger = +5.5 J/(mol . K)), a negative change in zero-point energy (Delta H-0 = -10.4 kJ/mol), and vibrational energy (Delta H-vib = +2.11 kJ/mol), giving rise to a negligible free energy of activation (Delta G double dagger = +0.34 kJ/mol). After formation of the alpha-agostic pi-complex the insertion proceeds without any large barriers directly to a beta-agostic product via a gamma-agostic primary product. The barrier of insertion is 2.0 kJ/mol and barrier for the product reorientation amounts to 2.5 kJ/mol. The overall reaction is exothermic by -98.2 kJ/mol. The propagation barrier corresponds to the rearrangement prior to insertion and rotation takes place in a concerted fashion. H-exchange (chain termination) has a higher electronic barrier (Delta H double dagger = +28.2 kJ/mol), and an only slightly lower free energy of activation (Delta S double dagger = -4.5 J/(mol . K); Delta G double dagger = +24.2 kJ/mol). Rotation is favored over H-exchange because of the high flexibility of the pi-complex. This gives a ratio of 15000/1 (insertion/H-exchange) based on Maxwell-Boltzmann statistics. Backside insertion starts with the formation of a backside pi-complex and has an activation barrier of 28.4 kJ/mol (Delta S double dagger : = -16.1 J/(mol . K), Delta G double dagger = +32.5 kJ/mol).