In My Element: How did I land in a gold mine?

Abstract

The In My Element series celebrates the personal accounts from Chemistry – A European Journal Editorial Board members for the 2019 International Year of the Periodic Table. In this contribution, Pekka Pyykkö gives his story on gold. In late 1975 Jean-Paul Desclaux and I had developed a fully relativistic (or non-relativistic) quantum chemical approach for hydride molecules, MHn. It converged well at short M−H bond lengths so we needed a molecule in which that was indeed the case. It turned out that AuH was one, so we tried it and it worked. There was a sizable relativistic contraction of the Au−H bond length. Moreover, relativistic effects explained most of the differences from AgH to AuH. We had the audacity to say that “the chemical difference between silver and gold may mainly be a relativistic effect”1 (see Figure 1). Earlier band-structure studies on Au metal had indicated large relativistic effects2 but in a germ, there may have been a trace of originality in our coupling of chemical periodicity to relativity. That turned out to be a scientific gold mine. Because the shell structure of gold has a local maximum of valence-shell relativistic effects, which normally scale as Z2, that was a lucky start. The relativistic (R) and non-relativistic (NR) orbital energies of the diatomic AgH and AuH (a.u. is atomic units). Data from Ref. 1. Figure from Ref. 17. A decade later, when lecturing at the Technical University of Munich, I learned from Hubert Schmidbaur about a strange attraction between two or more closed-shell AuI cations.3 They had both crystallographic and NMR evidence for it. Both cations carried a positive charge and had no open-shell electrons to make covalent bonds with. Yet they enjoyed an “aurophilic attraction” comparable in strength with a good hydrogen bond. Our explanation for it was a dispersion-type van der Waals interaction.4 Although other mechanisms also may contribute, for example electrostatic multipole interactions or induction,5-7 this is at least a partial explanation. That dispersion-type interaction is also comparable for silver,8, 9 and thus not specific for gold. We are not aware of a known case in which a suggested 6s–6p–5d hybridization10 would be sufficient to yield an observable attraction. Moreover, certain main-group elements also show it. Not only is gold a laboratory of relativistic quantum chemistry. Also, quantum electrodynamic (QED) effects can be seen in the ionization potential and electron affinity of the Au atom by pushing the current theories to their limits.11 Predicting new species with Au was a third aspect. Two examples were the first noble-metal−noble-gas bond12 in AuXe+, observed in mass-spectroscopy, or the predicted nanocluster WAu12,13 also subsequently made.14 Among novel covalent bonds, take the newly prepared unbridged AuII−AuII bonds.15 Both homogeneous and heterogeneous catalysis involving gold have become vast fields. Partially inspired by this application, the studies of both naked and ligand-covered gold nanoclusters are being intensively studied.16 Thus, keeping an eye on the theoretical chemistry of gold is a very good idea.17-20 Position Professor Emeritus of Chemistry E-mail [email protected] Homepage www.chem.helsinki.fi/∼pyykko ORCID 0000-0003-1395-8712 Education University of Turku, Finland, M.Sc. 1964, Ph.D. 1967. Post-doc at Aarhus, Denmark 1968–69 Post-doc at Gothenburg, Sweden 1969–70 Awards A.I. Virtanen Prize 1997, E.J. Nyström Prize 1998 decorated by President of Finland (FVR R I, 1995) Humboldt Prize 2002, Palmes Académiques (France) 2009 Schrödinger medal 2012 Honorary Fellow of Chinese Chemical Society 2012 Research interests Quantum chemistry of heavy elements Recently: simple models for eutectics