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A 2019 study reported an unusual six-coordinate palladium complex: three hydrides and three magnesium-based ligands sit around one palladium atom in a nearly flat hexagonal arrangement. The structure was striking because octahedral and trigonal-prismatic shapes are the familiar patterns for six-coordinate transition-metal complexes. The atoms’ arrangement was established experimentally; how to describe some of the interactions that hold it together remains open to interpretation.
What makes the geometry unusual?
In many familiar six-coordinate transition-metal complexes, six ligands surround a central metal in an octahedral arrangement or a trigonal-prismatic one. In an octahedron, the ligands occupy positions around the metal in three dimensions. In a trigonal prism, they define two triangular faces joined by rectangular sides.
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Garçon and colleagues reported a different arrangement in a simple, isolated complex with one central palladium atom: three hydride ligands alternate with three magnesium-based ligands around palladium, making a roughly six-sided ring in a single plane. The authors described it as the first simple coordination complex in which six ligands bond to one transition metal in a hexagonal-planar arrangement. Hexagonal-planar patterns had been observed in other settings, including metallic phases, pores in coordination polymers and clusters containing more than one nearby transition metal; the distinction was this isolated complex centered on one palladium atom.
The phrase “predicted over 100 years ago” appeared in the headline framing of the coverage. The 2019 paper discusses the history of coordination chemistry and Alfred Werner, but does not establish a precise date for a specific prediction of this geometry. It is therefore best treated as historical framing, not as a precisely verified date in the paper.
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How did the researchers establish the structure?
The team prepared palladium complexes from a palladium precursor and a magnesium reagent, then characterized the resulting compounds using several complementary methods. Single-crystal X-ray diffraction located the atoms in the crystals. Because hydrides are difficult to locate by X-ray methods, the authors identified their positions using a difference-density map and checked the 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.
For the two reported hexagonal-planar complexes, the authors found that the angles around palladium sum to 360 degrees. The Mg–Pd–H angles ranged from 54(2)° to 67(2)°, averaging 60(2)°. The largest deviation of a ligand from the hexagonal plane was about 10°. These values describe the paper’s particular compounds and crystals, not a general rule for palladium complexes.
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The reported Pd–Mg distances were 2.550(1)–2.567(1) Å in complex 1a and 2.485(1)–2.497(1) Å in 1b. The reported Pd–H distances were 1.57(4)–1.76(4) Å, while Mg···H distances were 2.08(5)–2.43(4) Å. These measurements help describe the geometry and proximity of atoms; on their own, distances do not settle every question about the nature of a chemical bond.
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What does “hexagonal-planar” mean here?
“Hexagonal” describes the approximate six-position pattern around palladium, while “planar” means those ligand positions lie close to the same plane. The term refers to the coordination arrangement, not to a claim that the entire molecule is a perfect flat hexagon. In the reported structures, the measured departures from the plane were small but not zero.
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The geometry can be distinguished from the usual reference shapes by considering both the arrangement of the six ligands and whether they occupy one plane:
| Arrangement | Basic shape | Ligand positions |
|---|---|---|
| Hexagonal-planar | Approximately six-sided ring around one central metal | Close to one plane in the reported palladium structures |
| Octahedral | Six positions around a three-dimensional central metal | Not confined to one plane |
| Trigonal-prismatic | Two triangular faces joined as a prism around the metal | Three-dimensional arrangement |
These are distinct coordination geometries, not interchangeable labels. The study establishes the palladium complex’s near-planar arrangement; it does not show that the geometry gives the compound a particular practical property.
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Why did chemists debate the bonding?
Identifying where atoms sit and deciding how to describe the interactions between them are related but separate tasks. The structural measurements support a near-planar arrangement. The choice to call particular palladium–magnesium interactions bonds, and thus to name the coordination geometry, depends partly on a chemical model of those interactions.
The authors’ model places sigma-donating hydrides and sigma-accepting magnesium-based ligands alternately around palladium. Their calculations describe the Pd–Mg interactions as predominantly ionic, while also identifying donor–acceptor interactions involving palladium d orbitals and magnesium-derived acceptor orbitals. They argued that those interactions, together with the measured distances and structural evidence, support describing the arrangement as hexagonal-planar. Their calculations also indicate weak residual magnesium–hydride interactions in this form.
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As Chemistry World reported in 2019, chemist Gregory Girolami questioned whether magnesium centers should be regarded as bonded to palladium: he suggested they might instead be attracted electrostatically to negatively charged hydrides attached to palladium, pointing to related work on iron hydrides. Study author Mark Crimmin acknowledged ionic contributions but argued that the structural and computational evidence supported the authors’ description. That disagreement concerns interpretation of bonding and terminology; it does not mean the measured atomic arrangement was fabricated, nor does the reporting establish that the terminology question has since been settled.
What might the finding lead to?
The work expands the range of coordination arrangements demonstrated for a simple transition-metal complex and may offer a design principle for chemists exploring unusual metal–ligand structures. The paper presents that as a possible implication, not as a demonstrated technology or commercial application. Its contribution is a structural chemistry result: a distinctive arrangement, supported by crystallographic and other measurements, alongside a continuing discussion of how to interpret the bonding.
The paper, “A hexagonal planar transition-metal complex,” by Martí Garçon and colleagues, was published in Nature on 9 October 2019 (volume 574, pages 390–393). Read the paper in Nature. The accepted manuscript and experimental details are available through UCL Discovery; crystallographic data were deposited with the Cambridge Crystallographic Data Centre, and computational and NMR data are identified in the paper’s data availability statement.
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