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

One unit of S(IV) (SO 2 or SHO 3 − ) 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 − d [Ni III ] T / dt =2 k [Ni III ] T [S(IV)] T where 2 k ={ k a [H + ] + k b K + k   c XK ′ X [H + ] [ X − ] + k   d XK ′ X K [ X − ]} {[H + ] + K } −1 {1 + K ′ X [ X − ]} −1 , here k a =87 ± 7 M −1 s −1 , k b =(2.5 ± 0.5)×10 3 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) (H 2 O) 2 ] 3+ with SO 2 and SHO 3 − , respectively. Monohalo species apparent equilibrium constants K ′ Cl =(1600 ± 400) M −1 and K ′ Br =(190 ± 20) M −1 and rate constants k   c Cl =80 ± 8 M −1 s −1 and k   c Br= 140 ± 15 M −1 s −1 are ascribed to the protonated pathway, involving the [Ni III (cyclam) (H 2 O) X ] 2+ and SO 2(aq) reaction pairs. The other two rate constants of k   d Cl =(5 ± 1)×10 3 M −1 s −1 and k   d Br =(3.1 ± 0.5)×10 4 M −1 s −1 , refer to the deprotonated pathway and are assigned to the [Ni III (cyclam) (H 2 O) X ] 2+ /SHO 3 − redox couple. A deuterium H 2 O / D 2 O 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.