The potential energy surfaces of the oxidative addition reaction CpML + CH4 --> CpML(H)(CH3) (Cp = eta(5)-C5H5; M = Rh, Ir; L = CO, PH3, SH2) have been studied at the MP2/LANL 1DZ and MP4SDTQ/LANL2DZ// MP2/LANLlDZ levels of theory. It has been found that there should be two competing pathways in those reactions, which can be classified as a sigma or pi approach, with the former being more favorable. A qualitative model which is based on the theory of Press and Shaik (Su, M.-D. Inorg. Chem. 1995, 34, 3829) has been used to develop an explanation for the barrier heights. As a result, our theoretical findings suggest that the singlet-tripler splitting (Delta E-st = E-triplet - E-singlet) of the CpML (M = Ph, Ir) species can be a guide to predict its reaction activity for oxidative additions. Considering the substituent effect and the nature of the metal center, the following conclusions therefore emerge: for the 16-electron CpML complex, a stronger electron-donating ligand as well as a heavier transition-metal center (such as Lr) will result in a smaller Delta E-st and, in turn, will be a potential model for the oxidative addition of saturated C-II bonds. Conversely, a better electron-withdrawing ligand as well as a lighter transition-metal center (such as Rh) will lead to a larger Delta E-st and then will be a good model for reductive coupling of C-H bonds. The results obtained are in good agreement with the available experimental results and allow a number of predictions to be made.