Successive oxidation of transition metal(II) aqua complexes (MIIOH2 to MIIIOH) is a domain in which proton-coupled electron transfer reactions are extremely common. The mechanism of these PCET reactions—concerted or stepwise—is an important issue in the understanding and design of natural or artificial systems catalyzing the formation of dioxygen by four-electron oxidation of water. Concerted proton-coupled electron transfer from an aqua metal(II) to a hydroxo metal(III) complex requires the close proximity of a proton-accepting group with a pK value between those of the aqua complexes. Otherwise, stepwise electron–proton or proton–electron pathways involving high-energy intermediates are followed. Concerted proton–electron pathways involving water as proton-acceptor or proton-donor group are inefficient. Cyclic voltammetry of the title complex in buffered aqueous solution and re-examination of previous results for the same complex attached to an electrode surface are used to establish these conclusions, which provide a starting point on the route to higher degrees of oxidation, such as those involved in the catalysis of water oxidation.