Reactions of [NiIII(cyclam)] with aqueous sulfur dioxide: Exhibition of a deuterium isotope effect and catalysis by chloride and bromide

One unit of S(IV) (SO₂ or SHO₃^?) is oxidized per 2 units of Ni^III(cyclam)] species to obtain sulfate. Kinetic analyses have been done by varying the acidities (0.013 ? H⁺] ? 1.0 M) and halide concentrations (0.000 ? X^?] ? 0.012 M; X=Cl and Br) at constant ionic strength (μ = 1.0 M). The rate law that incorporates the X^?] and H⁺] dependence is ?dNi^III]_T/dt=2kNi^III]_TS(IV)]_T where 2k={k_aH⁺] + k_bK + kK′_XH⁺] X^?] + kK′_XKX^?]} {H⁺] + K}^?1 {1 + K′_XX^?]}^?1, here k_a=87 ± 7 M^?1 s^?1, k_b=(2.5 ± 0.5)×10³ M^?1 s^?1 and pK = 1.8 ± 0.2. Rate constants k_a and k_b are attributed to the reactions of Ni^III(cyclam) (H₂O)₂]³⁺ with SO₂ and SHO₃^?, respectively. Monohalo species apparent equilibrium constants K′_Cl=(1600 ± 400) M^?1 and K′_Br=(190 ± 20) M^?1 and rate constants k=80 ± 8 M^?1 s^?1 and k = 140 ± 15 M^?1 s^?1 are ascribed to the protonated pathway, involving the Ni^III(cyclam) (H₂O)X]²⁺ and SO_2(aq) reaction pairs. The other two rate constants of k=(5 ± 1)×10³ M^?1 s^?1 and k=(3.1 ± 0.5)×10⁴ M^?1 s^?1, refer to the deprotonated pathway and are assigned to the Ni^III(cyclam) (H₂O)X]²⁺ /SHO₃^? redox couple. A deuterium H₂O / D₂O isotope effect of 2.1–2.8 can be attributed partially to an equilibrium isotope effect at low acidity though a small kinetic isotope (2.5 ± 0.5) effect is evident for the dihydrogen sulfito pathway, k_a. The kinetic isotope effect and the absence of sulfite radical scavenging effects are explained by a mechanism entailing migration of a hydride from sulfur to the Ni^III center to produce a Ni^III—H species, which rapidly comproportionates, and S(VI). © 1993 John Wiley & Sons, Inc.