Photosynthetic reaction centers are molecular photovoltaic structures that are of significant interest in the field of nanoscale science and technology. We report here the results of an extensive experimental survey and characterization of over 20 species of transition-metal compounds for their activity in the Hill reaction at the lipid-water interface of Photosystem II (PSII) membrane fragments. Four new PSII Hill reagents were identified: PtCl4, [PtCl6](2-), [OsCl6](2-), and [RuCl6](2-). Each can be photosynthetically reduced by PSII. The negative charge of the metal compound species is apparently essential for their interaction with the reducing site of PSII When PtCl4 is dissolved in water, it combines with water molecules to form H-2[PtCl4(OH)(2)], which can dissociate to the negatively charged species [PtCl4(OH)(2)](2-) at neutral pH. These findings could have significant implications for photosynthesis, nanoscale science, and nanofabrication in the field of biometallocatalysis.