Redox behavior of small metal clusters with respect to hydrogen. The effect of the cluster charge from density functional results

Tetrahedral model iridium species [Ir₄H_n]^q+ of different charge and hydrogen loading were described at the density functional level. The energy of dissociative adsorption of hydrogen was calculated to vary in the small interval from -63 kJ mol^-1 to -77 kJ mol^-1 (per H atom). Adsorption of hydrogen on Ir₄ and Ir₄⁺ induces an oxidation of the metal moiety, whereas the highly charged cluster Ir₄³⁺ is reduced upon hydrogen adsorption. The ligand shell acts as charge buffer as the metal moieties of the complexes [Ir₄H₁₂]^q+ with maximum hydrogen loading carry very similar effective charges, irrespective of the total charge q. Similar effects were confirmed to occur on small clusters of other 4d and 5d transition metals.