We report the results of an extensive experimental survey and characterization of over 20 species of transition-metal compounds for their activity in the Hill reaction and photoprecipitation of metal at the lipid-water interface of photosynthetic thylakoid membranes. Four new Hill reagents were identified : PtCl4, OsCl3, [RuCl6](2-), and [RuCl6](3-). Each can be photosynthetically reduced to form a metallic catalyst at the reducing site of photosystem I (PSI) in the thylakoid membranes. Negative charge of the metal-compound species is apparently essential for their interaction with the reducing site of PSI. When PtCl4 and OsCl3 an dissolved in water, they combine with water molecules to form H-2[PtCl4(OH)(2)] and H-3[OsCl3(OH)(3)], which can dissociate to negatively charged species [PtCl4(OH)(2)](2-) and [OsCl3(OH)(3)](3-) at neutral pH. Metallic ruthenium can also catalyze reduction of protons for H-2 production, in addition to its known catalytic ability to fix CO2. These findings could have important implications for nanofabrication and biometallocatalysis to harness the power of photosynthetic systems.