The Ziegler-Natta ethylene insertion into the metal-methyl bond in the neutral, bimetallic compound AlH2(mu-Cl)(2)TiCl2(CH3) has been studied with reaction path calculations and gradient geometry optimizations. No symmetry constraints have been imposed. The reaction path calculations and the geometry optimizations have been performed at the SCF level of theory using extended atomic basis sets. Correlation effects of all valence electrons have been included at the important stationary points on the potential energy surface (PES). A cut through the PES along the reaction path from the pi complex to the propyl product for the direct insertion mechanism is presented in detail, as well as equilibrium structures of reactants and the transition state of ethylene pi coordination. The system is seen to prefer geometries of C-s symmetry, and only departs from symmetry on short sections along the reaction path in order to relieve strain through rotations of the polymer chain. At the correlated level, the PES is almost flat along the reaction path until the transition state of ethylene insertion as determined at the SCF level of accuracy. Beyond this point, the energy decreases along the reaction path, resulting in an overall reaction enthalpy of 21.9 kcal/mol. The two bridging chlorides are found to behave dynamically, with an interchange in the bond distance asymmetry from the reactant to the product taking place synchronously with a polymer chain flipping due to the migratory insertion.