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– 1s– 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.