A polypyridylruthenium complex with an NAD(+)/NADH model ligand, [Ru(bpy)(2)(pbn)](2+) [bpy = 2,2'-bipyridine, pbn = 2-(2-pyridyl)benzo[b]-1,5-naphthyridine] in a D2O/CH3CN/triethylamine solution, undergoes stereospecific hydrogenation to give Delta-(S)- and Lambda-(R)- [Ru(bpy)(2)(pbnDD)](2+) [pbnDD = 5,10-dideutero-2-(2-pyridyl)benzo[b]-1,5-naphthyridine] upon visible-light irradiation. This result clearly indicates the pathway via the pi-stacked dimer of the deuterated one-electron-reduced species. The reduction of [Ru(bpy)(2)(pbn)](2+) with Na2S2O4 in D2O did not afford any stereospecific products. Furthermore, the more sterically crowded Ru complex, [Ru(dmb)(2)(pbn)](2+) (dmb = 6,6'-dimethyl-2,2'-bipyridine), did not produce the corresponding pbnDD species upon irradiation.