DI-MU-OXO-BIS[OXO(L-(+)-CYSTEINATO)MOLYBDATE(V)](2-)-DIOXYGEN REACTION

The reaction of di-μ-oxo-bis[oxo(L-(+)-cysteinato)molybdate(V)](2-), Mo2O4(cys)22-, I≡Mov2(cys)2, with O2 has been studied in HPO42--H2PO4-and HN3-NH4+ buffers. In HPO42--H2PO4- buffer at pH 6.5, excess O2 reacted with I according to the mechanism [FORMULA OMMITED]where at least one phosphate is bound to Mov2(cys)2. When a steady-state condition is imposed on MoIV(cys), the rate law becomes [FORMULA OMMITED]C. In excess I, the disappearance of O2 follows strict first-order kinetics under all conditions. The rate constant kobsd decreases with increasing [O2] (10-6-10-4 M) and increases with increasing [I], the best linear plot of the data being Kcobsd-1 vs. [I]-1. A mechanism is proposed involving a Mov2(cys)2-02 complex which yields MoVI and H202. The resulting H2O2 can oxidize Mov2(cys)2 or be catalytically decomposed. The excess I-O2 reaction was investigated at pH 9.8 in NH3-NH4+ buffer. It was found that NH3 enhances the reactivity of I toward O2. The rate law is assumed to be rate=[FOMULA OMMITED] It should be noted that kobsd for the reaction is also dependent upon [O2]init. The enhancement of I-O2 reaction by the bases phosphate, ammonia, and hydroxide is explained as the addition of these bases to I, which causes a coordination asymmetry, resulting in an asymmetry in oxidation state: IV-VI vs. V-V. Such asymmetric species can be reactive toward dioxygen and possibly dinitrogen. © 1979, American Chemical Society. All rights reserved.