Kinetic Study of the Formation of the Oxo Diperoxo Complexes of Molybdenum(VI) and Tungsten(VI) and their Reduction by Iron(II), Europium(II), the Methyl Viologen Radical Cation, and the Dithionite Ion.

摘要

Molybdenum (VI) rapidly combines with hydrogen peroxide in acidic, aqueous solution to form an oxo diperoxo complex with a large formation constant. There is substantial evidence that the structure of this complex is pentagonal bipyramidal, with an apical oxo and two eta(2) equatorial peroxo ligands, we will write the complex MoO(O2)2(H2O)2 as MoO(O2)2, and its hydrolyzed form (pK(a)=1.85) MoO(OH)(O2)2(H2O)(-) as MoO(OH)(O2)2(-). Tungsten (VI) forms an analogous but more acid complex (pK(a)=0.1); quantitative data for its rate of formation and formation constant are not available due to complexities in the chemistry of tungsten (VI) in acidic solution. In the present study, we have employed the substrates iron(II), europium(II), dithionite ion, and the methyl viologen radical cation (MV.+). These ions usually function as one-electron reducing agents, and with the exception of europium(II), the kinetics of their reactions with hydrogen peroxide have been reported. Masarwa et al. have proposed that one-electron reductions of hydrogen peroxide must proceed by an inner-sphere mechanism. The obvious extension to the oxo diperoxo complexes is the requirement of inner-sphere pathways for both oxygen atom transfer and one-electron reductions. It is therefore of interest to see whether marked activation of peroxide is operative toward the latter substrates as well. Reprints. (aw)