Mixed-valency in multinuclear nickel complexes: From fundamentals to nickel enzymes

Mixed-valence multi-metallic complexes, in which the metal is present in more than one oxidation state, provide crucial insight into how electron transfer operates in both biological proteins/enzymes and synthetic inorganic compounds. Nature offers striking examples, such as the oxygen-evolving complex (OEC) of photosystem II, where mixed valency plays an essential role in facilitating proton-coupled electron transfer (PCET) The degree of electronic delocalization between redox sites is subdivided into three groups (Class I, Class II, and Class III) by the Robin-Day classification. Elucidating electronic structure and the function of such systems serves as a foundation for the design of bioinspired catalysts. Nickel, with its rich redox flexibility, is well positioned to form mixed-valent binuclear complexes across several oxidation states, including Ni₂(I,0), Ni₂(I,II), and Ni₂(II,III) dinuclear complexes. Several such systems mirror the redox profiles of enzymes like acetyl-CoA synthase, which is central to C1 metabolism. This perspective highlights the emerging landscape of multinuclear nickel complexes, focusing on their structural classification and redox behavior. Special attention is given to a newly characterized family of Class III Ni₂(I,II) complexes, which exhibit fully delocalized valency. Collectively, this work underscores how mixed-valent states not only advance our understanding of electron transfer mechanisms but can also guide the development of new redox-active materials for catalysis.