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In 2019, chemists reported a simple transition-metal complex with six ligands arranged around one palladium atom in a nearly flat hexagon—an arrangement unusual beside the familiar octahedral and trigonal-prismatic forms. The structure was characterized with diffraction and spectroscopy, but the interpretation of how its magnesium-based ligands bond to palladium prompted scientific debate. The paper establishes the unusual structure; it does not demonstrate a practical application.
What does “hexagonal-planar” mean in this complex?
The complex reported by Martí Garçon and colleagues contains one central palladium atom surrounded by six ligands: three hydrides and three magnesium-based ligands. They alternate around palladium in an approximately planar hexagonal arrangement. “Hexagonal-planar” describes this coordination geometry: the six ligand positions form a near-hexagon in one plane around the metal.
For six-coordinate transition metals, octahedral and trigonal-prismatic arrangements are the standard reference geometries. Hexagonal-planar coordination had precedents in metallic phases, coordination-polymer pores and clusters with multiple nearby transition metals. The authors described their result as the first simple coordination complex with six ligands bonded to a single central transition metal in this arrangement. Garçon et al., Nature (2019)
How did the researchers establish the structure?
The team prepared palladium complexes from a palladium precursor and a magnesium reagent, then characterized them using several complementary methods. Single-crystal X-ray diffraction provided the atomic arrangement; the researchers located hydride positions from a difference-density map and checked those assignments with density functional theory (DFT) calculations. The study also reports neutron diffraction, multinuclear NMR spectroscopy, molecular-orbital analysis and quantum theory of atoms in molecules (QTAIM) calculations. Author accepted manuscript, UCL Discovery
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For the particular crystals studied, the reported Mg–Pd–H angles ranged from 54(2)° to 67(2)°, averaging 60(2)°. The angles around palladium summed to 360° in both complexes 1a and 1b, and the greatest deviation of a ligand from the hexagonal plane was about 10°. These are measurements of those compounds, not universal values for palladium complexes.
The authors reported Pd–Mg distances of 2.550(1)–2.567(1) Å in complex 1a and 2.485(1)–2.497(1) Å in 1b. For the reported hexagonal-planar structures, Pd–H distances were 1.57(4)–1.76(4) Å, while Mg···H distances were 2.08(5)–2.43(4) Å. The manuscript’s data-availability statement identifies crystallographic data deposited with the Cambridge Crystallographic Data Centre and computational and NMR data in a public repository.
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Why did chemists disagree about the bonding?
Finding atoms in a particular arrangement and deciding how best to describe their bonds are related but distinct tasks. Diffraction establishes atomic positions and supports the near-planar geometry. The nature of the interactions—and whether “bonded” is the most useful description for every ligand—requires a chemical model.
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1Clear out junk files and repair common Windows errors2Scan for outdated or missing drivers - takes under a minute3Repair Windows errors before they cause bigger problemsGarçon and colleagues modelled the arrangement as alternating sigma-donating hydrides and sigma-accepting magnesium-based ligands. Their calculations characterize Pd–Mg interactions as predominantly ionic, while proposing donor–acceptor interactions between palladium d orbitals and magnesium-derived acceptor orbitals as part of the case for the hexagonal-planar coordination description. They also found weak residual magnesium–hydride interactions in the planar form.
As Chemistry World reported, Gregory Girolami argued that magnesium centres could instead be electrostatically attracted to negatively charged, palladium-bound hydrides, pointing to related iron-hydride work. Mark Crimmin acknowledged ionic contributions but defended the authors’ interpretation based on the calculations and structural distances. This is a disagreement about how to interpret the interactions, not a dispute that the measured structure exists or evidence that the structural measurements are invalid.
Was this geometry really predicted more than 100 years ago?
The phrase “predicted over 100 years ago” appeared in the headline framing of the 2019 news coverage. The primary paper discusses how understanding of coordination-complex shapes developed from the work of Alfred Werner, but it does not establish a precise date for a specific prediction of this hexagonal-planar geometry. It is therefore safer to treat the century-old prediction wording as historical framing rather than a verified, precisely dated claim.
What could the result mean for chemistry?
The unusual structure may offer a design principle for creating other unusual coordination environments. That is a potential implication, not a demonstrated technology or commercial use: the 2019 paper reports a structural chemistry finding and does not show that the complex has led to a practical application.
Where to find the original study
The paper, “A hexagonal planar transition-metal complex,” by Martí Garçon, Clare Bakewell, George A. Sackman, Andrew J. P. White, Richard I. Cooper, Alison J. Edwards and Mark R. Crimmin, appeared in Nature on 9 October 2019, volume 574, pages 390–393. Its DOI is 10.1038/s41586-019-1616-2. A bibliographic record is available from PubMed.
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