The kinetics of reduction of the mer-[Ru III (pic) 3 ] complex (pic -= picolinato) by ascorbic acid (AscH 2 ) leading to formation of a red ruthenium(II) species have been studied spectrophotometrically by using both conventional mixing and stopped-flow methods. The reaction was followed as a function of the reductant concentration over a wide pH range (1.0-7.4). Electron transfer proceeds by an outer-sphere mechanism involving three protolytic forms of ascorbic acid, [a]2529 AscH 2 , AscHand Asc 2-, for which specific rate constants have been determined. The Gibbs' energy of activation was found to correlate linearly with the HOMO energies of the protolytic forms of the reductant. The mer-[Ru III (pic) 3 ] complex is too sparingly soluble in water to inhibit the growth of the Escherichia coli (ATCC 8739) strain. Its cytotoxicity against non-tumorigenic cells precludes its potential use as an anticancer agent.Scheme 1. Acid/base properties of the different redox forms of lascorbic acid. [6]

A detailed kinetic study of the reduction of cis‐dichloridobispicolinatoruthenate(III) by l‐ascorbic acid that leads to the formation of the corresponding RuII complex, was carried out spectrophotometrically using the stopped‐flow technique. The reaction was studied as a function of [AscH2]T, [RuIII], and pH. The observed kinetic traces could only be fitted to a three‐exponential function, characteristic for three parallel reaction paths. However, an earlier study clearly showed that under the selected experimental conditions only a single complex species is present in solution. The observed complication is suggested to arise from the low driving force of the reaction during which the re‐oxidation of RuII by the semi‐oxidized l‐ascorbic acid, i.e. ascorbyl radical Asc·–, accounts for the apparent three‐exponential behavior of the reaction. A detailed account of different modeling efforts and a comparison with related systems is presented.

The mer-[Ru(pic) 3 ] isomer, where pic is 2-pyridinecarboxylic acid, undergoes base hydrolysis at pH [ 12. The reaction was monitored spectrophotometrically within the UV-Vis spectral range. The product of the reaction, the [Ru(pic) 2 (OH) 2 ] -ion, is formed via a consecutive two-stage process. The chelate ring opening is proceeded by the nucleophilic attack of OH -ion at the carbon atom of the carboxylic group and the deprotonation of the attached hydroxo group. In the second stage, the fast deprotonation of the coordinated OH -ligand leads to liberation of the monodentato bonded picolinate. The dependence of the observed pseudo-first-order rate constant on [OH -] is given by k obs1 ¼ k þ k 1 ½OH À þk þ k 2 K 1 ½OH À 2 k À þk 1 þ k þ þk 2 K 1 ð Þ ½ OH À þk þ K 1 ½OH À 2 and k obs2 ¼ k ca þk cb K 2 ½OH À 1þK 2 ½OH À for the first and the second stage, respectively, where k 1 , k 2 , k -, k ca , k cb are the first-order rate constants and k ? is the second-order one, K 1 and K 2 are the protolytic equilibria constants.

Azure B (AB) and thionine (TH) cationic phenothiazyne dyes undergo an oxidative degradation by cerium(IV) in sulfuric acid media. TLC analysis of the reaction mixture containing an excess of the oxidant has shown the presence of thionine and/or methylene violet (Bernthsen) and other unidentified species. The kinetics of (i) two AB oxidation steps, namely the formation of a colored radical and its further decomposition, and (ii) the TH disappearance was studied using visible absorption spectroscopy combined with a stopped-flow method. Generation of the AB radical was confirmed by continuous flow electron paramagnetic resonance (EPR) spectroscopy. The reactions are first order in the total dye concentration, [S]. The kinetics of the studied systems is described by a common general rate law:

Oxidation of trispicolinatoruthenate(II) complex by hydrogen peroxide leads to the formation of mertrispicolinatoruthenium(III) in acidic or neutral solutions. Kinetics of the reaction were studied under a large excess of H 2 O 2 at constant pH. The initial rate method gives a rate expression of the form: Àd½RuðIIÞ=dt ¼ k II ½H 2 O 2 ½RuðIIÞ but the overall process examined till completion is far more complex. The rate of the reaction decreases with increasing pH to be practically completely retarded in alkaline media. The key step in the proposed reaction mechanism is the picolinato chelate ring opening followed by the substitution of the coordinated water by H 2 O 2 and two-electron intramolecular ruthenium(II) oxidation. Formation of the final ruthenium(III) complex is assigned rather to the ruthenium(IV) reduction by H 2 O 2 than ruthenium(II)-ruthenium(IV) comproportionation. The obtained results show the much slower rate of the trispicolinatoruthenate(II) oxidation by hydrogen peroxide or dioxygen than the mertrispicolinatoruthenium(III) reduction by such bioreductants as cytochrome cII or some cobalt(II) reductants.