Hydrogen bonds dethrone copper's dominance in metal stability rankings

For decades, copper has held an unquestioned place at the top of the stability hierarchy among transition metals. The Irving–Williams series, an empirical rule that ranks how firmly metals like manganese, iron, cobalt, nickel, copper, and zinc bind to surrounding molecules, has consistently shown copper winning out. This dominance was thought to be rooted in copper's electronic structure—the fundamental way its electrons are arranged—making it an intrinsic property of the metal itself, immutable and unchangeable. A team at KAIST has now upended that assumption.

Professor Yunjung Baek and her group developed a metal complex using a ligand based on the flavin framework found in vitamin B2. The ligand is the molecule that grips the metal at its center, and the team kept this direct bonding environment identical across all six metals they tested, from manganese through zinc. What they changed was not the metal's immediate surroundings but rather the weak hydrogen bonds in the outer shell—forces that hold the overall structure in place but do not directly touch the metal itself. These hydrogen bonds are ordinarily thought of as minor players in determining stability, background forces rather than decisive ones.

What the researchers discovered was that copper's legendary stability rests partly on a trick: the metal has a distinctive tendency to slightly reshape its bonding geometry into a form that favors its own stability, much like someone shifting their posture to find comfort. In the flavin-based structure the team designed, however, the hydrogen-bonded framework surrounding the metal acted as a constraint, preventing copper from adopting that preferred distorted shape. Locked into a geometry it did not favor, copper lost the additional stabilization it would normally gain through structural distortion. The result was an anti–Irving–Williams trend—copper no longer dominated.

The significance of this finding extends well beyond dethroning copper. What the work actually demonstrates is that the relative stability of metal complexes, long regarded as determined by each metal's intrinsic electronic properties, can be engineered by manipulating the environment around the metal. This opens a new design strategy. If a desired metal can be made to bond more strongly while others bond more weakly within a mixture, the principle could form the basis for systems that selectively extract or recover specific metals from complex solutions. The same logic applies to catalyst design: by tuning the surrounding environment, researchers could make a desired metal perform more effectively for a particular chemical reaction.

The approach also mirrors how living systems work. Proteins and enzymes in the human body routinely select the exact metal they need—iron, copper, zinc—from among many options present in the bloodstream. The KAIST findings suggest a new method for designing biomimetic systems that replicate these biological operating principles, allowing chemists to engineer artificial systems that achieve similar selectivity and function. The work was published in the Journal of the American Chemical Society on September 3, and Haneul Im, the study's first author and a combined master's and PhD student at KAIST, presented the research at the International Conference on Coordination Chemistry in Denmark, where she was the only Korean student to receive a Best Poster Award.

Professor Baek emphasized that the key point is not simply that copper's stability was lowered, but that the order of bonding stability itself—long regarded as an inherent property of each metal—can be changed through environmental design. This shift in understanding could reshape how chemists approach problems of metal separation, recognition, and catalysis, moving from accepting the metals' intrinsic properties as fixed constraints to treating the surrounding chemical environment as a powerful design tool.