Aluminum amidinate species, found to be active in ethylene polymerization, have been studied using a variety of computational methods, including semiempirical (AMI), Hartree-Fock and density functional theory type calculations, and first-principles MD simulations. In agreement with recently reported experimental observations, we find that for all pairs of experimentally studied substituents, dinuclear amidinate structures are very stable toward decomposition. However, with respect to the structure of the active ethylene catalyst, for the most stable dinuclear structures, sterically crowding substituents inhibit insertion through a very high-energy barrier, whereas for noncrowding systems, chain termination by beta -hydrogen transfer is likely to dominate over insertion. From finite temperature dynamics simulations, we observe strong fluctuations in the length of the bond bridging the two amidinate rings. It is suggested that the lengthening of that bond relaxes the steric constraints, lowering the barrier for insertion while still forcing the growing alkyl chain to adopt an orientation which inhibits rapid chain termination. Thus, effects explicitly related to finite (nonzero) temperature seem necessary to account for the catalytic activity of these amidinates. Finally, the present study clearly indicates that it is necessary to model the real catalyst, including bulky substituents if present, to arrive at a proper understanding of structure and activity.