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

Abstract

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 102 to 105 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 = 102–105, 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)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)6 3–, Mo­(CN)8 3–, or IrCl6 3– 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.