The potential energy surfaces of the oxidative addition reaction CpML + CH 4 → CpML(H)(CH 3 ) (Cp = η 5 -C 5 H 5; M = Rh, Ir; L = CO, PH 3, SH 2 ) have been studied at the MP2/LANL1DZ and MP4SDTQ/LANL2DZ// MP2/LANL1DZ levels of theory. It has been found that there should be two competing pathways in those reactions, which can be classified as a σ or π approach, with the former being more favorable. A qualitative model which is based on the theory of Pross 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−triplet splitting (Δ E st = E triplet − E singlet ) of the CpML (M = Rh, 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 Ir) will result in a smaller Δ E st and, in turn, will be a potential model for the oxidative addition of saturated C−H bonds. Conversely, a better electron-withdrawing ligand as well as a lighter transition-metal center (such as Rh) will lead to a larger Δ 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.
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Su et al. (1997) studied this question.
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