Most photochemical reactions degrade a metal complex, but recent exper
iments in this laboratory reveal a series of unusual photochemical rea
ctions that increase the metal content of the complex. A 450 nm excita
tion of bis(2,2'-bipyridine) (2,3-bis(2-pyridyl) pyrazine)ruthenium(II
), Ru(bpy)(2)dpp(2+), complexes in the presence of PtCl62-, PdCl62- an
d RhCl63- results in quantitative formation of the corresponding bimet
allic [Ru(bpy)(2)(dpp)MCl4](2+). The excited state energies and therma
l redox potentials of the chlorides and the specificity of the reactio
n suggest that formation of the bimetallic is not the consequence of e
nergy transfer or electron transfer quenching. Instead; we propose tha
t these reactions are the result of an excited state acid-base chemist
ry that manifests itself as changes in coordination and nucleophilicit
y. 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 stat
e. Population of the MLCT state (bpy)(2)Ru(CN)(2), on the other hand,
shifts charge density to the bpy ligand thereby decreasing the basicit
y of the cyanide ligand by ca. four orders of magnitude. The pH depend
ence of the quantum yield of decomposition of trans-[(bpy)(2)Ru(CN)(mu
-CN)Rh(NH3)(4)Br](2+) suggests that this immense change in basicity le
ads to a dissociative excited state. In complexes that exhibit excited
slate acid-base properties, the data presented here suggest that quen
ching is not necessarily limited to electron or energy transfer, but a
lso occurs by an excited state acid-base chemistry that leads to chang
es in coordination and/or nucleophilicity. (C) 1998 Elsevier Science S
.A.