A DFT Study of the Mechanism of the Spontaneous Activation of H2 by Ni, Pd, Pt, and Pd2

The bending energy profiles of 1 A 1 and 3 B 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 σ 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( 1 S 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 ( 3 B 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 σ 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.