OXIDATIONS OF HYDROGEN-PEROXIDE BY BIS(1,4,7-TRIAZACYCLONONANE)NICKEL(III), BIS(1,4,7-TRITHIACYCLONONANE)IRON(III) AND TRIS(2,2'-BIPYRIDINE)RUTHENIUM(III) IONS IN ACIDIC AQUEOUS-SOLUTIONS

The oxidations of hydrogen peroxide by typical outer-sphere oxidizing reagents, ,7-triazacyclononane)nickel(III)[Ni(tacn)(2)(3+)], 4,7-trith iacyclononane)iron(III)[Fe(ttcn)(2)(3+)], and s(2,2'-bipyridine)ruthen ium(III)[Ru(bipy)(3)(3+)], were studied in acidic aqueous solution [io nic strength I=0.10 mol dm(-3)(NaClO4)]. The stoichiometry was determi ned iodometrically as 2:1([M-III]:[H2O3]) for the reactions of [Ni(tac n)(2)](3+) and [Fe(ttcn)(2)](3+). Kinetic measurements with an excess of H2O2 revealed that HO2- is the only redox-active species for the re action with [Ni(tacn)(2)](3+) under the conditions 2 < -log[H+]<5.5, w hile the participation of both HO2- and H2O2 was observed for the reac tions with [Fe(ttcn)(2)](3+) and [Ru(bipy)(2)](3+). The second-order r ate constant for the oxidation reaction of HO2- by Ni(tacn)(2)(3+) was (6.15+/-0.06) x 10(6) dm(3) mol(-1) s(-1) at 25 degrees C with Delta H-double dagger=46.9+/-0.4 kJ mol(-1) and Delta S-double dagger=42+/-1 J K-1 mol(-1). The rate constants for the oxidation of HO2- and H2O2 by [Fe(ttcn)(2)](3+) at 25 degrees C were (2.63+/-0.18) x 10(9) dm(3) mol(-1) s(-1) with Delta H-double dagger=26.7+/-4.1 kJ mol(-1) and Del ta S-double dagger=25+/-14 J K-1 mol(-1), and 3.12+/-0.03 dm(3) mol(-1 ) s(-1) with Delta H-double dagger=40.1+/-0.9 kJ mol(-1) and Delta S-d ouble dagger=-101+/-3 J K-1 mol(-1), respectively. The rate constants for the oxidation reaction of HO2- and H2O2 by [Ru(bipy)(3)](3+) at 25 degrees C were (3.70+/-0.26) x 10(7) and 3.57+/-0.19 dm(3) mol(-1) s( -1), respectively It was confirmed from the Marcus-type cross-relation that the oxidations of HO2- by [Fe(ttcn)(2)](3+) and [Ru(bipy)(3)](3) proceed through the outer-sphere mechanism with k(ex)(HO2.-HO2-)=3.8 x10(-3) dm(3) mol(-1) s(-1). The second-order rate constant for the ox idation of HO2- by [Ni(tacn)(2)](3) was ca. 10(3) times faster than th e predicted rate constant from the other outer-sphere reactions. The k inetic isotope effect (kH/k(D)=2.8) indicates that the reaction of HO2 - with [Ni(tacn)(2)](3+) proceeds via a transition state (shown in Sch eme 1). The difference in the reactivity of small ions is discussed in relation to the electronic coupling in the precursor complex and the solvation structures of small ions.