Manganese, iron, cobalt, and nickel oxo-, peroxo-, and superoxoclusters: a density functional theory study

The 3d-transition-metal dioxo-, peroxo-, and superoxoclusters with the general composition MO2, M(O2), and MOO (M = Mn, Fe, Co, and Ni) were studied by DFT by the B1LYP functional. The dioxides in their ground states represent the global minima for the M + O2 system. Both ground-state dioxides and the lowest-energy peroxides are in their (d-only) highest spin states. The 6A1 state of Co(O2) exceeds the d-only spin-multiplicity value (quartet), being nearly isoenergetic with the 4A1 state of Co(O2). The energy gain on transforming the peroxides to the corresponding dioxides decreases in the order Mn(O2) > Fe(O2) > Co(O2) > Ni(O2) and varies in the range 0.27-1.8 eV. The dissociation energy to M + O2 for all studied peroxides is less than 1 eV being the lowest (0.47 eV) for Mn(O2). The Mn and Fe peroxides need less than 0.3 eV to rupture one of the MO bonds to form the corresponding superoxide. Mn and Fe superoxides are less stable than the corresponding peroxides; the superoxide of Co is more stable than its peroxide, while Ni superoxide is unstable--its energy is above the limit of dissociation to Ni + O2. According to the electrostatic potential maps, the oxygen atoms in the peroxides are more nucleophilic than those in the dioxides and superoxides, in which the terminal oxygen atom is more nucleophilic than the M-bonded oxygen atom. This result differs from the expectations based on charge-distribution analysis.