Photosystem II electron transfer: the manganese complex to P680.

bridges in transition metal clusters are unreactive as a class, they would require special activation if they were to be involved in oxidations or 0-0 bond formation. We propose that the unusual electronic configuration of the Mn, cluster of the WOC is an expression of this activation. Formation of the activated (dn)3(dx2-,2)' electron configuration may also be explained if the Mn"' ions have only five ligand atoms, arranged so that the equatorial ligand field is weaker than the axial component. This geometry has been observed in the five-coordinate complex, HR(Pz(3,5-i-Pr),l3Mn(p-O),Mn[Pz( 3,5-i-Pr),],BH + , which also has the unusual Mn"' hyperfine anisotropy as seen in the WOC. Five-coordinate Mn"' ions are rarely encountered in chemistry. They are susceptible to hydrolysis and act as strong oxidizing agents. These features offer a satisfying explanation for the unique reactivity of the WOC as catalyst for water oxidation.