The rates of nickel(II)–murexide complex formation were studied in dimethylsulfoxide/nitromethane mixed solvents at 25 °C with the latter being an "inert" solvent, to elucidate the mechanism of the reaction. Results do not fit in detail with a D mechanism and the five-coordinated intermediate interpretation. Id mechanisms are able to explain the observations with and without invoking a nitromethane-containing intermediate, Ni(DMSO)5NM2+. Both mechanisms were tested with the aid of nmr data on solvation (second coordinate sphere nitromethane population). Ligand solvation unavoidably plays a major role in the determination of the encounter as well as the complex formation equilibrium. The simplest mechanism assumes an Id process in which the probability of encounter between Ni(DMSO)62+ and murexide ion is quite sensitive to solvent composition.
