Experimentally measured rate constants, k(12)(obsd), for the reductions of [Ni(III)tripeptides(H2O)(2)] with Fe(CN)(6)(4-), Mo(CN)(8)(4-), and W(CN)(8)(4-) are 10(2) to 10(5) times faster than the calculated rate constants with the Marcus theory for outer-sphere electron-transfer processes, k(12)(calc), even when work terms are considered. This gives rise to a kinetic advantage of k(12)(obsd)/k(12)(calc) = 10(2)-10(5), which is consistent with an inner-sphere electron-transfer mechanism via a bridged intermediate. In addition, k(12)(obsd) values are nearly independent of the electrochemical driving force of the reactions. This is consistent with one of the two axial water ligands coordinated to [Ni(III)tripeptides(H2O)(2)] being substituted in the rate-limiting step to form bridged intermediates of the type [(CN)(5or7)M-(CN)-Ni-III(tripeptide)(H2O)(4-) with M = Fe-II, Mo-IV, or W-IV. A limiting rate constant of H2O replacement from [Ni(III)tripeptides(H2O)(2)] of (5 +/- 2) X 10(7) M-1 s(-1) at 25.0 degrees C is observed. Electron paramagnetic resonance spectra of Ni(III) peptide complexes in the presence of Fe(CN)(6)(3-), Mo(CN)(8)(3-), or IrCl63- provide evidence for the cyanide-bridged intermediates. Substitution-limited electron-transfer reactions could serve as an additional criterion for inner-sphere pathways when atom or group transfer does not occur during electron-transfer and when precursor and successor complexes cannot be observed directly.