A computational study of oxidative addition (OA) of methane to Re(OC2H4)(3)A (A = ancillary ligand, which thus may interact with the metal) was carried out. The choice of ancillary ligands has been made based on their electronic properties: A = B or Al (Lewis acid), 04 or SiH (electron precise), N (sigma-donor), and P (sigma-donor/pi acid): The main objective of this study was to understand how variation in A affects the structural and electronic properties of the reactant d(4)-Re(III) complex, which can ultimately tune the kinetics and thermodynamics of OA. Results obtained from MP2 calculations revealed that, for OA of CH4 to Re(OC2H4)3A, the order of Delta G(double dagger) for a choice of ancillary ligand is B > Al > SiH > CH > N > P. Single point calculations for Delta G(double dagger) obtained with CCSD(T). showed excellent agreement with those computed with MP2 methods. MCSCF calculations indicated that oxidative addition transition states are well described by a single electronic configuration, giving further confidence in the MP2 approach used for geometry optimization and Delta G(double dagger) determination, and that the transition states are more electronically similar to the d(4)-Re(III) reactant than the d(2)-Re(V) product.