Figure 2.

Sketches of skeletal mechanisms for formation of an O₂ molecule from two molecules of H₂O. Elevations in oxidation state of M are indicated by ‘+’, etc. Boxes indicate the crucial stages in each mechanism. (a,b) Ways of making an O–O bond (peroxo intermediate) that is relatively easy to oxidize further to give O₂. (a) Formation of peroxo intermediate by homonuclear combination from adjacent M⁺−OH units deprotonated and rearranged as two oxyl radicals. (b) Formation of peroxo intermediate by attack of H₂O on an electrophilic ‘O’ of [M·O]⁺⁺ group. (c) Interconversion between two O²⁻ that are either terminal ([M·O]⁺⁺) or bridging ((μ-O) between two M⁺⁺), an ${\text{O}}_2^{2-}$ bridged between two M⁺, and O₂ bound (weakly) between two M. (d) Attack of a bridging O²⁻ on an oxyl radical.