Redox Reactions between Two Metal Complexes

Abstract

The investigations of the kinetics and mechanisms of electron transfer between metal complexes are continuing to diversify, from the reactions of octahedral species in aqueous solution to systems involving less common metals and coordination geometries in nonaqueous media. The activity in the study of electron transfer reactions involving metalloproteins is increasing rapidly, with an emphasis on the rate dependencies on distance and free energy. The format of the Chapter remains much the same as in previous volumes, with sections covering metal aqua and oxo ions, metal ion complexes, and metalloproteins. The rate and activation parameters are presented in four tables, with the data for electron transfer reactions between metal complexes in Table 2.1, directly measured electron self-exchange rate data in Table 2.2, and the data for intramolecular and bimolecular reactions involving metalloproteins in Tables 2.3 and 2.4, respectively. The discussion in each section of the text and the data in the tables are arranged, for the most part, in the order of increasing atomic number of the central metal ion in the reductant. Reactions of Metal Aqua and Oxo Ions 2.2.1. Titanium{III) and (IV)The variety of kinetic behaviors observed in the reductions of [(NH 3 )sRuX](3-n)+ (X n -= SO~-, S20j-, and NH 2 S0 3 ) by Ti3+ has been interpreted in terms of the ability of sulfato ligands to mediate electron transfer.(1) Outer-sphere reductions occur when x n -= NH2S03 or SO~-, with Ti3+ and 27 M. V. Twigg (ed.), Mechanisms of Inorganic and Organometallic Reactions © Plenum Press, New York 1989 28 2 • Redox Reactions between Two Metal Complexes 2.2.3. Chromium(II} The reduction of the [M020iH20 )6]2+ ion by a large excess of Cr2+ proceeds via an intermediate {Mo(IIIh[ 4Cr(IIII)]} which is formed rapidly (kl = 9.1 X 10 3 M-3 S-I) with the rate law (2) and decays slowly (k2 = 2.0 X lO-s M-I S-I) to Rate = k l [Cr2+]2[Mo(V)2][H+] (2) the Mo(IIIh product.(7) A second intermediate {Mo(IVh[2Cr(III)]} is observed when the reaction is carried out at a Cr: Mo ratio of 2: 1, leading to the production 2.2 • Reactions of Metal Aqua and Oxo Ions 29 of[M0 3 0 4(H20)9]4+. This Mo(IVh trimer is reduced with excess Cr+ in a two-stage process, the first of which is an inner-sphere reaction with an inverse acid dependence. The dimolybdate ions [Moz(j. L-X)(JL-Cl)2CI6]3-(X = H, Cl) undergo a two-electron reduction by Cr+ in acidic chloride solution to yield the [M02Cls]4ion, which catalyzes the anaerobic oxidation of Cr+ to Cr(I1I).(S)The product of the reduction of[ Co(bpy h]3+ by Cr+, upon aerial oxidation, is a red dimeric species, postulated to have the structure [(H20)4Cr(JL-OH)2Cr(OH2)2]4+.(9) This product and the stoichiometry of the reaction suggests a two-electron process, with the bpy ligand serving as a temporary bridging radical. An investigation of the Cr(I1) reduction of [Co(pdh] (pd = pentane-2,4-dione) in water/acetone mixtures reveals outer-sphere, and mono-and di-bridged ([H+r l dependent) pathways. (10) The effect of the cosolvent ...