(CN) 5 3-,2-and LH = benzotriazole or benzimidazole) were prepared and characterized in aqueous solutions by means of electrochemical and spectroelectrochemical methods. Special emphasis was given to the pH-dependent redox processes, exhibited by all the investigated complexes. From their related Pourbaix diagrams, which displayed a typically Nernstian behavior, the pK a and formal reduction potential values were extracted. In addition, these E 1/2 versus pH curves were also used to illustrate the partitioning relationship concerning the redox and acid-base species, and their interconversion equilibria. The active area in which the dependence of the M III /M II couple on the pH takes place, as delimited by pK a III and pK a II , was taken into account in order to evaluate the usefulness of such simple complexes as models for proton-coupled electron transfer (PCET). The results were interpreted in terms of the acceptor/ donor electronic character of the ligands and σ,π-metal-ligand interactions in both redox states of the metal ion.