Repulsion, not attraction, found to be the force in metal-metal interaction

20 May 2021

A research team led by Professor Che Chi-Ming (Croucher Senior Research Fellowship 1997) and Dr Yang Jun, from the Research Division for Chemistry and Department of Chemistry at the Faculty of Science, University of Hong Kong, has resolved a long-standing fundamental problem in the field of metal-metal closed-shell interaction.

The work has been published in the Proceedings of the National Academy of Sciences (PNAS).

Metal-metal closed-shell interaction, also known as metallophilicity, has a significant role in diverse fields of chemistry, such as supramolecular chemistry and organometallic chemistry.

For example, metallophilicity is important in the fabrication of self-assemblies by transition metal complexes, with demonstrated applications in organic semiconductors, bio-sensing, and functional optoelectronic materials.

In the microworld of small molecules, there are many types of interactions. Metallophilicity describes the interaction between metal atoms as having closed-shell electronic configurations.

In the early 1970s, chemists observed an interesting phenomenon: two closed-shell metal atoms could form a short metal-metal distance. A special “attraction” was proposed to exist between two metal atoms that would push them closer together.

Many theoretical models were put forward to account for this, such as orbital hybridisation or the relativistic effect of heavy metal atoms, such as gold or platinum. However, it has remained controversial, with conflicting observations in experiments continuing to perplex inorganic and theoretical chemists.

Now, Che and Yang, together with postdoctoral fellow Dr Wan Qingyun, and co-workers have concluded that metallophilicity is not, after all, an attractive interaction in an organometallic complex, but actually repulsive in nature.

This is due to strong M-M’ Pauli repulsion, which they proved from their high-level calculations and experimental techniques.

The research team concluded that orbital hybridisation and the relativistic effect would strengthen metal-metal Pauli repulsion when forming a close metal-metal contact. Intermolecular dispersion and electrostatic interaction counterbalance this repulsion, leading to a short metal-metal distance.

This theoretical model could also explain why the Ag-Ag distance is shorter than the Au-Au distance, given the weaker Ag-Ag Pauli repulsion induced by less orbital hybridisation in the Ag complex.

The research will provide a new theoretical perspective on the possibility of making new supramolecular materials with inexpensive earth-abundant transition metal complexes, such as palladium, silver, or nickel.

Professor Che Chi-ming is Zhou Guangzhao Professor in Natural Sciences and Director of the State Key Laboratory of Synthetic Chemistry at the University of Hong Kong (HKU). Che received his BSc (1978) and PhD (1982) degrees from HKU. He conducted research at the California Institute of Technology from 1980 to 1983, returning to Hong Kong to join HKU’s Department of Chemistry in 1983. He was promoted to Chair Professor in 1992. From 1999-2016, he served as the Dr Hui Wai-Haan Chair of Chemistry. He was awarded a Croucher Senior Research Fellowship in 1997.

To view Professor Che’s Croucher profile, please click here