Intramolecular conversion of pentaaquahydroperoxidochromium(III) ion to aqueous chromium(V): potential source of carcinogenic forms of chromium in aerobic organisms.

The decomposition of the title compound (H(2)O)(5)CrOOH(2+) (hereafter Cr(aq)OOH(2+)) in acidic aqueous solutions is kinetically complex and generates mixtures of products (Cr(aq)(3+), HCrO(4)(-), H(2)O(2), and O(2)). Relative yields of individual products vary greatly with reaction conditions and initial concentrations of Cr(aq)OOH(2+). At pH 5.5 in the presence of O(2), the reaction was complete in less than a minute and generated chromate in about 70% yield. These findings, in addition to poor reproducibility of kinetic data, are indicative of the involvement of one or more reactive intermediates that consume additional amounts of Cr(aq)OOH(2+) in post-rate-determining steps. The kinetics were simplified in the presence of H(2)O(2) or ABTS(2-), both of which are capable of scavenging strongly oxidizing intermediates. The measured rate constant in 0.10 M HClO(4) at low O(2) concentrations (≤0.03 mM) was independent of the concentration of the scavengers and was, within error, the same for ABTS(2-), k = 4.9 (±0.2) × 10(-4) s(-1), and H(2)O(2), k = 5.3 (±0.7) × 10(-4) s(-1). At a constant ionic strength of 1.0 M, the reaction in the presence of either H(2)O(2) or ABTS(2-) obeyed a two-term rate law, k(obs)/s(-1) = 6.7 (±0.7) × 10(-4) + 7.6 (±1.1) × 10(-4) [H(+)]. Both in the presence and absence of ABTS(2-) as the scavenger, the yields of H(2)O(2) increased with increasing [H(+)]. These results are discussed in terms of a dual-pathway mechanism for the decay of Cr(aq)OOH(2+). The H(+)-catalyzed path leads to the dissociation of H(2)O(2) from Cr(III), while in the H(+)-independent reaction, Cr(aq)OOH(2+) is transformed to Cr(V). Both scavengers rapidly remove Cr(V) and simplify both the kinetics and products.