Three chloro(4'-(N-methylpyridinium)-2,2': 6', 2 ''-terpyridine) platinum(II) (abbreviated as PtL2+) derivatives tethered to a single alkyl viologen unit (-(CH2) n-CH2-N+C5H4-C5H4N+-CH3; abbreviated as -Cn-MV2+, where n = 1, 3, and 4), i. e., PtL2+-Cn-MV2+, have been synthesized and investigated in detail. It is shown that the turnover number (TON) for the photocatalytic H-2 evolution from water in the presence of a sacrificial electron donor EDTA (ethylenediaminetetraacetic acid disodium salt) is dramatically improved by the attachment of a single alkyl MV2+ unit (TON = 21.5-25.2, 12 h). Spectrophotometric studies reveal that the photoirradiation of PtL2+-Cn-MV2+ in the presence of EDTA initially leads to the formation of a 1-electron-reduced species, and then to a 2-electron-reduced species, where reductive quenching of a photoexcited species is a major path to the reduced photoproduct in each step. Electrochemical studies show that two consecutive 1-electron reductions at the PtL2+ unit are nearly overlapped with the corresponding reductions at the MV2+ unit. The 1-electron-reduced species can be thus expressed as either PtL+--Cn-MV2+ or PtL2+-Cn-MV+-, while the 2-electron-reduced one as PtL2+-Cn-MV+-. Moreover, the latter products behave as stacked species involving three types of p-dimer sites, (PtL+)(2), (MV+)(2), and (PtL+) (MV+), and do not drive thermal H2 evolution according to the reaction: PtL+center dot-Cn-MV+center dot + 2H(+) -> PtL(2+)Cn- MV2+ + H-2. The H-2 evolution from water photocatalyzed by PtL2+-Cn-MV2+ has been found to occur via formation of 3-electron-reduced species; PtL+center dot-Cn-MV+center dot + EDTA + h nu -> PtL0-Cn-MV+center dot (or PtL+center dot-Cn-MV0) + EDTA(ox), and PtL0-Cn-MV+center dot (or PtL+center dot-Cn-MV0) + 2H(+) -> PtL+center dot-Cn-MV2+ (or PtL2+-Cn-MV+center dot) + H-2.