The kinetics of a fast leuco-Methylene Blue (LMB) re-oxidation to Methylene Blue (MB) by copper(II)–halide (Cl⁻, Br⁻) 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(II) species (and H⁺(aq)), and the pseudo-first order rate constant (k′_obs) is largely independent of the atmosphere used (air, oxygen, argon). The rate law, at constant Cl⁻ (Br⁻) anion concentration, is given by the expression: (d[MB⁺])/dt = ((k_aK[H⁺] + k_b)/(1 + K[H⁺]))·[Cu^II][LMB] = k′_obs[LMB], where K is the protonation constant, and k_a and k_b are the pseudo-second order rate constants for protonated and deprotonated forms of LMB, respectively. The rate law was determined based on the observed k′_obsvs. [Cu^II] and [H⁺] dependences. The rate dramatically increases with [Cl⁻] over the range: 0.1–1.5 M, reflecting the following reactivity order: Cu²⁺(aq) ≪ CuCl⁺(aq) < … < CuCl₄ ²⁻. The slow re-oxidation of LMB by oxygen has also been briefly examined at different [H⁺]. ESR results provide clear evidence for the formation of an intermediate radical. The mechanistic consequences of all these results are discussed.

Ligand substitution reactions of [Ru^II(terpy)(N^N)Cl]Cl, where N^N = bipy and en, and the corresponding aqua complexes were studied in detail, including the crystal and solution structures of the chlorido complexes. The chlorido complex undergoes spontaneous aquation that can only be prevented using high chloride concentrations (>2 M).