The potential energy surfaces for the reactions MCH2+ + H-2 --> M+ + CH4 (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 RhCH2+ + H-2 --> Rh+ + CH4 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 RhCH2+ compound. Further shift of the hydrogen atom toward the carbon leads to formation of the hydride-methyl complex (HRhCH3+). Changing of the singlet spin state probably occurs immediately after formation of the HRhCH3+ 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.