ELECTRON-TRANSFER .119. REDUCTIONS OF RHODIUM-BOUND SUPEROXIDE

Treatment of (Rh(II))24+(aq) with 02 in 3 M HClO4 Yields a violet (eps ilon560max = 620 M-1 cm-1) superoxo complex, the properties of which a re in accord with the assigned formula [(H2O)4(OH)Rh(III)(O2)Rh(III)(O H)(H2O)4]3-. This cation is reduced, in 1:1 reactions with V(II), Eu(I I), Ti(III), and Fe(II), or in 2:1 reactions with ascorbic acid and hy droquinone, to a le reduction product, the corresponding peroxo comple x. The latter regenerates the superoxo derivative, together with the b ridged cation, [(H2O)4Rh(mu-OH)2Rh(OH2)4]4+, on standing. The yield of superoxo complex reformed in this disproportionation approaches the t heoretical 67% with the Eu(II) reaction, but falls between 45 and 57% with the other reductants. Reductions of the superoxo complex utilize parallel pathways, at different protonation levels. In most cases, an inverse-[H+] route competes with an [H+]-independent path; with Ti(III ) and V(III), kinetic components are proportional to [H+]-1 and [H+]-2 . The high rate of reduction by V(II) insures that this reaction proc eeds without ligand bridging and, in conjunction with the model of Mar cus, allows us to estimate a self-exchange rate of 10(-3.2) M-1 S-1 fo r the (Rh(III))2-bound superoxo-peroxo couple. Analogous consideration of the Fe(II) reduction, however, points to a predominant inner-spher e path in this instance. The regeneration of the superoxo from the per oxo complex exhibits first-order kinetics with a rate constant 2.1 X 1 0(-3) s-1 (23-degrees-C, mu = 2.0 M) independent of the reductant at h and or the acidity of the medium, suggesting a rate-determining hetero lysis of the peroxy function. The 2e- reagents, Sn(II), U(IV), and (Mo 2O4)2+ also react with equimolar quantities of the superoxo complex. T he product here is ESR-silent at room temperature and, unlike the le p roduct, does not regenerate the parent superoxo species on standing.