Water-bound metal (M) complexes play a central role in the catalytic centers of natural systems such as Photosystem II (PSII), superoxide dismutase, cytochrome c oxidase and others. In these systems, electron transfer reactions involving the metal center are coupled to proton transfers. Besides its fundamental interest, comprehension of these reactions and of possible bio-inspired catalytic devices is an additional motivation for studying the coupling between proton and electron transfer (proton-coupled electron transfers, PCET), starting with an aqua-M(II)/hydroxo-M(III) couple, and going to higher oxidation degrees as in the case of PSII (hydroxo-M(III)/oxo-M(IV) couple). Factors that determine the occurrence of the stepwise and concerted pathways are recalled from the illustrating example of a recently described mononuclear osmium complex, thus opening perspectives for further studies of the biomimicking complex. PCET in a mononuclear aqua/hydroxo manganese couple was then studied using the electrochemical approach.