The kinetics of a fast leuco-Methylene Blue (LMB) re-oxidation to Methylene Blue (MB) by copper()-halide (Cl Ϫ , Br Ϫ ) complexes in acidic aqueous media has been studied spectrophotometrically using a stopped-flow technique. The reaction follows a simple first order rate expression under an excess of the copper() species (and H ϩ (aq)), and the pseudo-first order rate constant (kЈ obs ) is largely independent of the atmosphere used (air, oxygen, argon). The rate law, at constant Cl Ϫ (Br Ϫ ) anion concentration, is given by the expression:where K is the protonation constant, and k a and k b are the pseudosecond order rate constants for protonated and deprotonated forms of LMB, respectively. The rate law was determined based on the observed kЈ obs vs. [Cu II ] and [H ϩ ] dependences. The rate dramatically increases with [Cl Ϫ ] over the range: 0.1-1.5 M, reflecting the following reactivity order: Cu 2ϩ (aq) Ӷ CuCl ϩ (aq) < . . . < CuCl 4 2Ϫ. The slow re-oxidation of LMB by oxygen has also been briefly examined at different [H ϩ ]. ESR results provide clear evidence for the formation of an intermediate radical. The mechanistic consequences of all these results are discussed.

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 series of cis and trans complexes of the general formula [Cr(cyc)(OH 2 )X] 2ϩ , where cyc is the macrocyclic tetraamines meso-or rac -5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane and X Ϫ = NCS Ϫ , N 3 Ϫ and Cl Ϫ , have been prepared in solution via anation of the diaqua complexes. The kinetics of the monodendate X ligand release in alkaline media have been studied at a range of temperatures and hydroxide concentrations at ionic strengths of 1.0 or 2.0 M using NaClO 4 , NaBr or NaCl as "inert" electrolytes. A higher order than linear dependence of the pseudo-first-order rate constant on the hydroxide concentration has been observed for all the systems investigated. These results have been rationalized in terms of specific ion-pair interactions between the macrocyclic chromium() reactants and counterions of the supporting electrolytes, followed by proton transfer in the hydroxide ion pair to give a reactive conjugate base. DALTON

The oxidation of methylene blue (MB + ) by cerium(IV) was studied in 0.1-5 M H 2 SO 4 . The reaction proceeds via MB radical (MB 2+• ) formed by one electron transfer to the oxidant. The radical is observed spectrophotometrically by a very intense absorbance at k max = 526 nm and by the e.p.r signal at g = 2.000. The kinetics of the fast radical formation are two orders of magnitude slower than its decomposition, which were examined using a stopped-flow method at 298 K under pseudo-first order conditions. The rate laws for the both steps were determined and a likely mechanism reported.

The kinetics of hexacyanoferrate(III) reduction by hydrogen peroxide in strongly alkaline media leading to hexacyanoferrate(II) ion have been studied spectrophotometrically within the wavelength range 300-500 nm. The reaction obeys a simple pseudo-first-order rate expression under the applied conditions, namely, a large excess of the reductant and OH -anion concentrations, and a low oxidant concentration. The linear dependences of the pseudo-firstorder rate constant on OH -and H 2 O 2 concentrations are consistent with the rate law of the form: À d½Fe III dt ¼ ðk II HO 2 À þ k III O 2 2À ½OH À Þ½HO 2 À ½Fe III where k II HO 2 À and k III O 22À are the second-and the pseudo-third-order rate constants for the electron transfer from HO 2 -and O 2 2-to [Fe(CN) 6 ] 3-, respectively. The apparent activation parameters determined at 0.4 M NaOH are as follows: DH # = (18.0 ± 1.0) kJ mol -1 and DS # = (-155 ± 3.5) J K -1 mol -1 . The possible mechanism of the reaction is discussed.

Oxidation of the macrocyclic Cr(III) complex cis-[Cr(cycb)(OH)(2)](+), where cycb = rac-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane, by an excess of the hexacyanoferrate( III) in basic solution, slowly produces Cr(V) species. These species, detected using e.p.r. spectroscopy, are stable under ambient conditions for many hours, and the hyperfine structure of the e.p.r. spectrum is consistent with the interaction of the d-electron with four equivalent nitrogen nuclei. Electro-spray ionization mass spectrometry suggests a concomitant oxidation of the macrocyclic ligand, in which double bonds and double bonded oxygen atoms have been introduced. By comparison basic chromate(III) solutions are oxidized rapidly to chromate(VI) by hexacyanoferrate(III) without any detectable generation of stable Cr(V) intermediates. Kinetics of oxidation of the macrocyclic Cr(III) complex in alkaline solution has been studied under excess of the reductant. Rate determining formation of Cr(IV) by a second order process involving the Cr(III) and the Fe(III) reactants is seen. This reaction also involves a characteristic higher order than linear dependence on the hydroxide concentration. Reaction mechanisms for the processes, including oxidation of the coordinated macrocyclic ligand under excess of the oxidant- are proposed

The Cu II -mediated oxidation of promazine by dioxygen to form promazine 5-oxide was studied in the presence of a large excess of dioxygen, Cu II -halides (Cl ) , Br ) ) and H + ions using u.v.-vis and ESR spectroscopies. The first step is a fast reaction between promazine and Cu II -halides leading to the production of a stable promazine radical with much higher yield in bromide than chloride media. ESR results provide clear evidence for the formation of this radical. In the second step the cation radical is oxidized by dioxygen to a dication hydrolyzing to promazine 5-oxide. The promazine-superoxide complex, concentration of which is determined by steady-state approximation, is postulated as a significant intermediate resulting from the reduction of dioxygen by the cation radical. The final product, promazine 5-oxide, is formed via a spontaneous and a Cu II -assisted reaction path way. Cu II controls the reaction rate through: (i) oxidation of promazine to the promazine radical, (ii) acting as a scavenger of superoxide, and (iii) slow oxidation of the promazine radical in the parallel reaction. The rate is independent of [H + ], linearly dependent on [O 2 ] and only slightly dependent on [Cu II ] within the excess concentration range of the Cu II complexes used. Mechanistic consequences of all these results are discussed.

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.