Electron transfer. 154. Reaction of the nitrosodisulfonate anion radical with one- and multi-electron inorganic reductants

摘要

The nitrosodisulfonate anion radical, ~·NO(SO_3)_2~(2-), a single electron oxidant, reacts cleanly with an array of metal-center reductants. Rates for substitution-labile species generally fall in the range 10~4-10~5 M~(-1)s~(-1), both for le~-[Eu~(II), Fe~(II)(EDTA)] and 2e~- (U~(IV), In~I, Ge~(II) and Sn~(II)) reagents. The more weakly reducing donor VO~(2+) reacts more slowly (3 M~(-1) s~(-1) at 22 ℃) and the substitution-inert complex Fe(CN)_6~(4-) still more sluggishly. Reductions with the nonmetal related donors, azide, formate, hypophosphite, hyponitrite and H_2AsO_3~-, fall in the range 10~(-4)-10~(-2) M~(-1)s~(-1). Reactions yield the product HON(SO_3)_2~(2-). Kinetic decay curves feature no irregularities reflecting formation or loss of one or more transients on a time scale comparable to that of the main conversion, indicating that the experimental rates pertaining to the 2e reductants and to hydrazine (a 4e reductant) are determined by the initial le transfer and that fellow-up steps are rapid. The usual distinction between complementary and noncomplementary redox reactions applies poorly to reductions of ~·NO(SO_3)_2~(2-). Rapid reductions require (1) an adequate potential difference, (2) the possibility of an inner-sphere path and (3) the availability of one or more nonbonding electrons at the reducing site. Substitution-labile metal-based reducing centers embracing a wide range of formal potentials, both le~- and 2e~- donors, may react speedily, although the latter group must operate in steps.