Classical trajectory calculations are performed to determine differences in the microscopic dynamics for two fundamental processes for the Cl a -+CH3Clb→ClaCH 3Clb - reactive system: Cla --CH3Clb complex formation and directly attaining the [Cla-CH3-Clb]- central barrier without first forming the complex. This latter process becomes important when the C-Clb stretch mode is excited in the CH 3Clb reactant. The total cross section for complex formation and directly attaining the central barrier increases as n C-Clb is increased. The value for the Cla --C-Clb angle θ as the reactants interact, the dynamical stereochemistry, is found to be a very important property for distinguishing between the mechanisms for the two fundamental processes. Directly attaining the central barrier requires oriented reactants with θ≈π, while orientation suppresses complex formation. Substantial reactant orientation only occurs for CH3Clb rotational temperatures less than 300 K. The complex is formed by a T→R energy transfer process which involves coupling between the reactant orbital angular momentum and CH3Clb rotational angular momentum. Complex formation does not involve energy transfer to the CH3Clb vibrational modes, which is consistent with an earlier finding that the CH 3Clb intramolecular modes are inactive during decomposition of the Cla --CH3Clb complex. Orientation of CH3Clb enhances coupling between the Cla -+CH3Clb radial motion and the C-Clb stretch mode. This coupling leads to the above direct substitution process and extensive deactivation of the excited C-Clb stretch during direct unreactive collisions. Considerably less deactivation results from Cla --CH3Clb complex formation followed by dissociation to the reactants. Rotationally exciting CH3Clb eliminates orientation and, thus, suppresses deactivation of the C-Clb stretch.