The excited state of tetrakis (mu-pyrophosphito-PP')diplatinate(LT), Pt-2(pop)(4)(4-), has been shown to be reductively quenched by a series of Ni(II) complexes. Steady-state photolysis of Pt2(pop)44- in the presence of Ni(cyclam)(2+) and SO42- ions was followed by absorption spectroscopy and showed the production of the Ni(III) complex with low efficiency. The Ni(III) yield was enhanced in the presence of oxygen presumably because of scavenging of Pt2(pop)(4)(5-) from the solvent-caged (Pt2(II,I)....Ni(III)) primary product pair resulting from electron transfer. Cage escape yields of 0.028 in N-2 Saturated solution and 0.054 in aerated solutions were estimated from the data. For a series of complexes with decreasing redox potential, the rate constants for their Pt-3(2)(pop)44- phosphorescence lifetime quenching decreased with driving force Delta G, consistent with Rehm-Weller behavior and leading to a reorganization energy of similar to 60 kJ mol(-1). The diffusional rate constant calculated from the Debye-Smoluchowski equation was 1.6 x 10(10) M-1 s(-1), in excellent agreement with the observed value of 1.6 x 10(10) M-1 s(-1) based on the Rehm-Weller equation. The transmission factor for the electron transfer was estimated at 10(-4), in the weakly adiabatic region. Although the Rehm-Weller treatment has been used successfully in reactions involving mainly uncharged organic reactants, in this investigation it leads to an unrealistic ratio for the rate constants for the diffusive separation of the charged reactants and products. An alternative interpretation of the results based on an approach by Meyer and Nagle removes this problem and the observed linear dependence of In k(q), on the square root of E-o (Ni(II)/Ni(III)) in the endergonic region shows that the dominant back electron transfer produces ground-state species. The analysis also leads to an estimate of the Pt-3(2)(pop)(4)(4-)/Pt-2(pop)45- potential of 1.29 +/- 0.05 V(vs NHE), in excellent agreement with the previous literature value of 1.34 V.