Superoxides three-electron bond

The second is a complex of sulfenium cation and superoxide and explains the near equivalence of the spin density on the two peroxyl oxygens in thiyl peroxyls. Both valence structures display an exposed electron-deficient sulfur that makes it susceptible to nucleophilic solvents. Complexing with a lone pair donor is expected to stabilize the charge-separated state increasing its contribution to the ground state. A partial radical-cationic character of the sulfur could also make three-electron bonding with some solvents contribute to the charge separation. 

Germacrene-D, a ten-membered-ring system with three double bonds acting as electron-rich centers, and pinenes and menthadiene oiXylopia aethiopica EO showed a significant ability to scavenge superoxide anion radical [176]. EOs with /f-caryophyllene as the major compound showed radical-scavenging activity [177]. 

In the superoxide ion, 02, there are three electrons occupying the In orbitals. The bond order is 1.5, which is consistent with its observed bond dissociation energy of 360 kJ mol-1 and bond length of 132 pm. 

It should be noted that when the 0-0 bond is broken, the O2 molecule bound at Fe + receives four electron equivalents and yields a water molecule and an oxide on Fe +, at least, formally. The stable oxygenated form and the unstable peroxide intermediate provide the four-electron reduction of O2 at once. As is well known, if an O2 molecule receives four electrons one at a time, three active oxygen species, superoxide, peroxide, and hydroxyl radical, will be produced during the O2 reduction. Cytochrome c oxidase must reduce O2 totally without releasing these species, which are extremely toxic to the cell. The four-electron reduction of this enzyme may be the strategy of this enzyme for safe O2 reduction (without damaging cells). 

The reaction continuum for HO can be subdivided into three discrete categories that are outlined in Scheme 14 (a) displacement reactions in which the leaving group departs with an electron supplied by HO (polar-group transfer), (b) addition reactions in which a covalent bond is formed (polar-group addition), and (c) simple electron-transfer reactions in which HO acts as an electron donor (single-electron transfer). This view of the chemistry of HO also applies to the reactions of superoxide ion (O2 ) and other nucleophilic oxyanions (Table 16). 

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