The role of inner-sphere configuration changes in electron-exchange reactions of metal complexes

Abstract

Extended X-ray absorption fine structure (EXAFS) techniques have been used to determine the differences in the metal–ligand bond distances Δd₀ for the following couples (couple, Δd₀/Å) Fe(H₂O)^{2 +/3 +} ₆, 0.13; Fe(CN)^{4 –/3 –} ₆, 0.01; Fe(phen)^{2 +/3 +} ₃, 0.00; Co(NH₃)^{2 +/3 +} ₆, 0.22; Co(bpy)₃ ^{+/2 +}, – 0.02; Co(phen)^{2 +/3 +} ₃, 0.19; Ru(H₂O)^{2 +/3 +} ₆, 0.08; Ru(NH₃)^{2 +/3 +} ₆, 0.04. The Cr(H₂O)^{2 +} ₃ ion has four short and two long Cr—O bonds (2.07 and 2.30 Å, respectively) and the average Δd₀ for the Cr(H₂O)^{2 +/3 +} ₆ couple is 0.20 Å. The rate constant for the Fe(H₂O)^{2 +} ₆/Fe(H₂O)^{3 +} ₆ exchange reaction is 1.1 dm³ mol^{– 1}s^{– 1} at 25 °C and 0.1 mol dm^{– 3} ionic strength. The exchange rate is independent of the cation (Li⁺/Na⁺) used to maintain constant ionic strength. Exchange rate constants for twelve reactions that span fifteen orders of magnitude are shown to be in excellent agreement with the semi-classical model.