The pK(a) values associated with protonation of the one-electron reduced fo
rms of series of [L'(2RuL)-L-II](2+) complexes {L' = bidentate polypyridyl
ligand; L = bidentate polypyridyl ligand with additional uncoordinated N at
oms in the aromatic ring system: e.g., dpp 2,3-bis(2-pyridyl)pyrazine, bpz
= 2,2'-bipyrazine} were assessed using pulse radiolysis techniques by the m
easurement of spectral variations as a function of pH. A linear correlation
was observed between pK(a) and E degrees(RuL'L-2(2+/+)) for complexes in w
hich the protonatable ligand was at the same time the site of reduction. In
complexes where one or more of the nonprotonatable ligands (L') had very l
ow pi* energy levels {e.g. (CF3)(4)bpy}, reduction occurs on a nonprotonata
ble ligand and a dramatic decrease in the pK(a) values was observed fur the
reduced species. In complexes where the energies of the protonatable and n
onprotonatable ligands were comparable, the protonation behavior was consis
tent with some orbital mixing/delocalization of the electronic charge.