The oxidative addition of the F-CH3 bond to coordinatively unsaturated trans-M(X)(PH3)(2) (M = Rh, Ir; X = CH3, H, Cl) was theoretically investigated by density functional theory. All of the stationary points were determined at the B3LYP/LANL2DZ level. A configuration mixing model based on the theory of Press and Shaik has been used to develop an explanation for the barrier height as well as the reaction enthalpy. Our theoretical findings suggest that the singlet-triplet splitting (Delta E-st = E-triplet-E-singlet) of the ML3 species can be used as a basis to predict its reaction activity for oxidative additions; i.e., the smaller the Delta E-st of ML3, the lower the barrier height and the larger the exothermicity, in turn, the faster the oxidative addition reaction. Considering the substituent effect, and the nature of the central metal, the following conclusions therefore emerge: for the 14-electron trans-(M(X)(PH3)(2) complex, a stronger pi-donor ligand (such as Cl) as well as a heavier transition metal center (the third-row) will result in a smaller Delta E-st, and thus will provide a potential model for the oxidative addition of saturated C-F bonds. In this work, an energetically feasible reaction mechanism which should not have radical intermediates involved is suggested.