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For more than a century, the well-known 18-electron rule has guided the field of organometallic chemistry. Now, researchers at Okinawa Institute of Science and Technology (OIST), in collaboration with scientists from Germany, Russia, and Japan, have successfully synthesized a novel organometallic compound that challenges this longstanding principle. They have created a stable 20-electron derivative of ferrocene, an iron-based metal-organic complex, which could lead to exciting possibilities in chemical science.

"For many transition metal complexes, they are most stable when surrounded by 18 formal valence electrons. This is a chemical rule of thumb on which many key discoveries in catalysis and materials science are based," said Dr. Satoshi Takebayashi, lead author of the paper in Nature Communications.

Ferrocene is a classic example that embodies this rule. "We have now shown for the first time that it is possible to synthesize a stable 20-electron ferrocene derivative," he added.

This breakthrough improves our understanding of the structure and stability of metallocenes, a class of compounds known for their characteristic "sandwich" structure, in which a metal atom sits between two organic rings.

Rebuilding our conceptual understanding

First synthesized in 1951, ferrocene revolutionized chemistry with its unexpected stability and unique structure, eventually earning its discoverers the 1973 Nobel Prize in Chemistry. In many ways, ferrocene opened a new chapter in our understanding of metal鈥搊rganic bonding and launched the modern field of organometallic chemistry, which continues to inspire generations of scientists to explore metal鈥搊rganic compounds.

This new study builds on that foundation. By designing a novel ligand system, the team was able to stabilize a ferrocene derivative with 20 valence electrons, coordination chemistry that was previously considered improbable. "Moreover, the additional two valence electrons induced an unconventional redox property that holds potential for future applications," Dr. Takebayashi noted.

This is important because even though ferrocene is already used in reactions involving electron transfer, known as redox reactions, it has traditionally been limited to a narrow range of oxidation states.

By enabling access to new oxidation states through the formation of an Fe鈥揘 bond in this derivative, it expands the ways in which ferrocene can gain or lose electrons. As a result, it could become even more useful as a catalyst or functional material across a variety of fields, from energy storage to chemical manufacturing.

Understanding how to break and rebuild the rules of chemical stability enables researchers to design molecules with tailor-made properties. These insights could inspire new research aimed at advancing sustainable chemistry, including the development of green catalysts and next-generation materials.

A platform for future innovation

Ferrocene derivatives have already made their way into various technologies, from solar cells and pharmaceuticals to medical devices and advanced catalysts. By expanding the conceptual toolkit available to chemists, this latest breakthrough could help build on and diversify these applications while inspiring entirely new ones.

The Organometallic Chemistry Group at OIST focuses on uncovering the fundamental principles that govern metal-organic interactions and applying them to real-world challenges. The team has a special interest in unconventional compounds that defy standard chemical rules, such as the 20-electron ferrocene derivative reported in this study.