The mechanistic pathways of formation of the NADH-like [Ru(bPY)(2)(pbnHH)](2+) species from [Ru(bPY)(2)(pbn)](2+) were studied in an aqueous medium. Formation of the one-electron-reduced species as a result of reduction by a solvated electron (k = 3.0 x 10(10) M-1 s(-1)) or CO2 center dot- (k = 4.6 x 10(9) M-1 s(-1)) or reductive quenching of an MLCT excited state by 1,4-diazabicyclo[2.2.2]octane (k = 1.1 x 10(9) M-1 s(-1)) is followed by protonation of the reduced species (pK(a) = 11). Dimerization (k(7a) = 2.2 x 10(8) M-1 s(-1)) of the singly reduced protonated species, [Ru(bPY)(2)(pbnH(center dot))](2+), followed by disproportionation of the dimer as well as the cross reaction between the singly reduced protonated and nonprotonated species (k(8) = 1.2 x 10(8) M-1 s(-1)) results in the formation of the final [Ru(bpy)(2)(pbnHH)](2+) product together with an equal amount of the starting complex, [Ru(bpy)(2)(pbn)](2+). At 0.2 degrees C, a dimeric intermediate, most likely a pi-stacking dimer, was observed that decomposes thermally to form an equimolar mixture of [Ru(bpy)(2)(pbnHH)](2+) and [Ru(bpy)(2)(pbn)](2+) (pH 9). The absence of a significant kinetic isotope effect in the disproportionation reaction of [Ru(bpy)(2)(pbnH)](2+) and its conjugate base (pH > 9) indicates that disproportionation occurs by a stepwise pathway of electron transfer followed by proton transfer.