Density Functional Study of the Mechanisms and the Potential Energy Surfaces of MCH2++ H2Reactions. The Case of Cobalt and Rhodium (M = Co, Rh)

The potential energy surfaces for the reactions MCH 2 + + H 2 → M + + CH 4 (M = Co, Rh) have been studied by means of a density functional approach in which we considered both the singlet and triplet state channels. The H−H bond cleavage in the singlet state follows a stepwise H 2 splitting corresponding to an oxidative hydrogen addition mechanism; while for the triplet state only a concerted mechanism with the formation of a four-center transition state structure is possible. For the reaction involving Co + cation, the triplet channel is energetically favored at the beginning and at the end of the reaction, although the barrier height for the H 2 activation is much lower in the excited singlet path. However, the energetic penalty required to obtain the singlet excited state is prohibitive and precludes this pathway as a channel for this reaction. Thus, we conclude that this reaction (M = Co + ) follows a four-center mechanism in the triplet state. On the basis of our results we conclude that the RhCH 2 + + H 2 → Rh + + CH 4 reaction should be a spin-forbidden process. The reaction starts in the singlet ground state and follows through an oxidative addition mechanism of H 2 to the Rh + moiety of the RhCH 2 + compound. Further shift of the hydrogen atom toward the carbon leads to formation of the hydrido−methyl complex (HRhCH 3 + ). Changing of the singlet spin state probably occurs immediately after formation of the HRhCH 3 + intermediate. This intermediate in the triplet state is metastable and collapses to the final complex without any barrier. Our results for both considered reactions are in good agreement with available experimental data.