Most photochemical reactions degrade a metal complex, but recent experiments in this laboratory reveal a series of unusual photochemical reactions that increase the metal content of the complex. A 450 nm excitation of bis(2,2 -bipyridine)(2,3-bis(2-pyridyl)pyrazine)ruthenium(II), Ru(bpy) 2 dpp 2 + , complexes in the presence of PtCl 2 - 6 , PdCl 2 - 6 and RhCl 3 - 6 results in quantitative formation of the corresponding bimetallic [Ru(bpy) 2 (dpp)MCl 4 ] 2 + . The excited state energies and thermal redox potentials of the chlorides and the specificity of the reaction suggest that formation of the bimetallic is not the consequence of energy transfer or electron transfer quenching. Instead, we propose that these reactions are the result of an excited state acid-base chemistry that manifests itself as changes in coordination and nucleophilicity. The emissive MLCT state in Ru(bpy) 2 (dpp) 2 + is localized on dpp, and H + quenching indicates that population of the emissive MLCT state increases the basicity of at least one of the peripheral nitrogens by at least four orders of magnitude relative to that in the ground state. Population of the MLCT state (bpy) 2 Ru(CN) 2 , on the other hand, shifts charge density to the bpy ligand thereby decreasing the basicity of the cyanide ligand by ca. four orders of magnitude. The pH dependence of the quantum yield of decomposition of trans-[(bpy) 2 Ru(CN)(μ-CN)Rh(NH 3 ) 4 Br] 2 + suggests that this immense change in basicity leads to a dissociative excited state. In complexes that exhibit excited state acid-base properties, the data presented here suggest that quenching is not necessarily limited to electron or energy transfer, but also occurs by an excited state acid-base chemistry that leads to changes in coordination and/or nucleophilicity.