A Kinetic Study of the Electron-Transfer Reactions of Nickel(III,II) Tripeptide Complexes with Cyano Complexes of Molybdenum, Tungsten, and Iron.

Experimentally measured rate constants, k12obsd, for the reductions of [Ni(III)tripeptides(H2O)2] with Fe(CN)64-, Mo(CN)84-, and W(CN)84- are 102 to 105 times faster than the calculated rate constants with the Marcus theory for outer-sphere electron-transfer processes, k12calc, even when work terms are considered. This gives rise to a kinetic advantage of k12obsd/k12calc = 102-105, which is consistent with an inner-sphere electron-transfer mechanism via a bridged intermediate. In addition, k12obsd 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)5or7M-(CN)-NiIII(tripeptide)(H2O)]4- with M = FeII, MoIV, or WIV. A limiting rate constant of H2O replacement from [Ni(III)tripeptides(H2O)2] of (5 ± 2) × 107 M-1 s-1 at 25.0 °C is observed. Electron paramagnetic resonance spectra of Ni(III) peptide complexes in the presence of Fe(CN)63-, Mo(CN)83-, 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.