The bending energy profiles of (1)A(1) and B-3(2) electronic states have been calculated for the M:H-2 (M = Ni, Pd, and Pt) compounds using three parameters functional (B3LYP). It has been shown that the sigma(H-H)-bond activation leading to the dihydride species in a singlet state is possible for the nickel and platinum atoms, while for Pd, the complex Pd-H-2, weakly bound with respect to Pd(S-1(0)) + H-2, is formed spontaneously. In the first case, Ni and Pt, the interaction beginning from the triplet state of the metal center correlates with the hydride species (B-3(2) state) after passing a high barrier height (46.5 and 11.5 kcal/mol, for Ni and Pt, respectively). An inter-system crossing favors the activation of the sigma(H-H)-bond in the singlet state ((1)A(1)). For Pd, another mechanism occurs. The dihydride species may be formed starting from the molecular complex ((1)A(1)) by a slightly endothermic process through low activation energy (6 kcal/mol). For the Pd-2 dimer, whose ground state is a triplet state, the dissociation of H-2 is similar to that of the Ni and Pt atoms. The mechanism implies an inter-system crossing between the singlet and triplet states. After the crossing point, a nonplanar mechanism implying a folding of the Pd-Pd-H-2 planes is proposed.