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).

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 \( k_{{{\text{obs}}1}} = \frac{{k_{ + } k_{1} [{\text{OH}}^{ - } ] + k_{ + } k_{2} K_{1} [{\text{OH}}^{ - } ]^{2} }}{{k_{ - } + k_{1} + \left( {k_{ + } + k_{2} K_{1} } \right)[{\text{OH}}^{ - } ] + k{}_{ + }K_{1} [{\text{OH}}^{ - } ]^{2} }} \) and \( \left( {k_{{{\text{obs}}2}} = \frac{{k_{ca} + k_{cb} K_{2} [{\text{OH}}^{ - } ]}}{{1 + K_{2} [{\text{OH}}^{ - } ]}}} \right) \) 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.

This review covers highlights of the work performed in the van Eldik group on inorganic reaction mechanisms over the past two decades in the form of a personal journey. Topics that are covered include, from NO to HNO chemistry, peroxide activation in model porphyrin and enzymatic systems, the wonder-world of Ru^III(edta) chemistry, redox chemistry of Ru(III) complexes, Ru(II) polypyridyl complexes and their application, relevant physicochemical properties and reaction mechanisms in ionic liquids, and mechanistic insight from computational chemistry. In each of these sections, typical examples of mechanistic studies are presented in reference to related work reported in the literature.