Application of Piszkiewicz model on the electron transfer reaction of dithionite ion and bis-(2-pyridinealdoximato)dioxomolybdate(IV) complex.

The need to better understand the binding mode of antioxidants (sulfur oxyanions) kinetically is a concern in medicine. Hence, a spectrophotometric method was used to study the application of the Piszkiewicz model on the electron transfer reaction of dithionite ion (S2O42-) and bis-(2-pyridinealdoximato)dioxomolybdate(IV) complex at 303 K and an absorption maxima of 560 nm. It follows an acid dependent reductive pathway that is medium sensitive. Charge distribution from the reaction species contributes to the redox efficiency of the system, resulting in a primary salt effect (NaCl) with an enhanced reaction rate. Alteration of the reaction medium with ethanol led to an elevation of reduction time as the charge catalysis was distorted by a drop in the system permittivity. Likewise, sodium dodecyl sulfate in the system decreased the reduction rate of the complex due to the low impact of hydrophobic and ion interaction between the micelle and substrates. First order reaction kinetics was observed in the concentration of the redox partners and a 2:1 (complex: S2O42-) stoichiometry was obtained with the involvement of hydrogenated sulfite radical which resulted in the formation of sulfur dioxide and a Mo2+ deactivated complex. The occurrence of counterion catalysis is pronounced in the reaction system owing to the participation of like-charged substrates in the rate-controlling phase. The standard enthalpy (69.12 [Formula: see text] 0.05 kJ mol-1) and Gibbs energy (80.10 [Formula: see text] 0.07) kJ mol-1 suggest that the process is endothermic dependent. The investigation of the anionic surfactant effect on the reaction medium was quantitatively ascertained from the Piszkiewicz model of the complex interaction sequence.

A kinetic study and mechanisms of reduction of N, N'-phenylenebis(salicyalideneiminato)cobalt(III) by L-ascorbic acid in DMSO-water medium.

The kinetics of reduction of N, N 1 -phenylenebis-(salicylideneiminato)cobalt (III), referred to as [Co(Salophen)]+ by L-ascorbic acid (H2A) was studied in mixed aqueous medium (DMSO:H2O; 1:4 v/v) under pseudo-first-order conditions at 33 ± 1 °C, µ = 0.1 mol dm-3 (NaCl) and λ max = 470 nm. L-ascorbic acid was oxidized to dehydroascorbic acid with kinetics that was first order in both the [H2A] and [Co(Salophen)+] and second-order overall. The reaction involves two parallel reaction pathways; an acid-dependent and the inverse acid-dependent pathways. The inverse acid pathway shows that there is a pre-equilibrium step before the rate determining-step in which a proton is lost. The kinetics followed negative Brønsted-Debye salt effect. Evidence was obtained for the presence of free radicals but none to support the formation of an intermediate complex of significant stability during the reaction. Overall, the data obtained suggest an outer-sphere mechanism for the reaction. A plausible mechanism is proposed.

Reaction of N, N 1 -phenylenebis(salicyalideneiminato)cobalt(III) and l-cysteine in mixed aqueous medium: kinetics and mechanism.

The redox kinetics involving the reaction of N, N'-phenylenebis(salicyalideneiminato)cobalt(III) ([CoSalophen]+) and l-cysteine (LSH) was studied using pseudo-first order approach under the following conditions, [H+] = 1.0 × 10-3 mol/dm3, µ = 0.1 C2 mol/dm3 (NaCl), λmax = 470 nm and T = 27 ± 1 °C in DMSO: H2O; 1:4 v: v medium. The redox reaction was 1st order in both [CoSalophen+] and [LSH], with the overall 2nd order. Hydrogen ion concentration effect revealed the activeness of both the protonated and deprotonated form of the reductant, positive Brønsted-Debye salt effect and was also ion catalyzed. There was no evidence suggesting an intermediate complex of significant stability in the reaction. Free radical was detected to take part and as such the reasonable mechanistic pathway for the reaction is suggested to be outer-sphere, hence proposed.