Two different four-electron reductions of dioxygen (O-2) on a metal surface are reproduced in homogeneous systems. The reaction of the highly unsaturated (56-electron) tetraruthenium tetrahydride complex 1 with O-2 readily afforded the bis(mu(3)-oxo) complex 3 via a dissociative mechanism that includes large electronic and geometric changes, i.e., a four-electron oxidation of the metal centers and an increase of 8 in the number of valence electrons. In contrast, the tetraruthenium hexahydride complex 2 induces a smooth H-atom transfer to the incorporated O-2 species, and the O-OH bond is cleaved to afford the mono(mu(3)-oxo) complex 4 via an associative mechanism. Density functional theory calculations suggest that the higher degree of unsaturation in the tetrahydride system induces a significant interaction between the tetraruthenium core and the O-2 moiety, enabling the large changes required for the dissociative mechanism.