A hexagonal planar transition-metal complex

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Abstract

Transition-metal complexes are widely used in the physical and biological sciences. They have essential roles in catalysis, synthesis, materials science, photophysics and bioinorganic chemistry. Our understanding of transition-metal complexes originates from Alfred Werner’s realization that their three-dimensional shape influences their properties and reactivity1, and the intrinsic link between shape and electronic structure is now firmly underpinned by molecular-orbital theory2,3,4,5. Despite more than a century of advances in this field, the geometries of transition-metal complexes remain limited to a few well-understood examples. The archetypal geometries of six-coordinate transition metals are octahedral and trigonal prismatic, and although deviations from ideal bond angles and bond lengths are frequent6, alternative parent geometries are extremely rare7. The hexagonal planar coordination environment is known, but it is restricted to condensed metallic phases8, the hexagonal pores of coordination polymers9, or clusters that contain more than one transition metal in close proximity10,11. Such a geometry had been considered12,13 for [Ni(PtBu)6]; however, an analysis of the molecular orbitals suggested that this complex is best described as a 16-electron species with a trigonal planar geometry14. Here we report the isolation and structural characterization of a simple coordination complex in which six ligands form bonds with a central transition metal in a hexagonal planar arrangement. The structure contains a central palladium atom surrounded by three hydride and three magnesium-based ligands. This finding has the potential to introduce additional design principles for transition-metal complexes, with implications for several scientific fields.

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Fig. 1: Preparation of hexagonal planar complexes.
Fig. 2: Preparation of group-10 hydride complexes with magnesium ligands.
Fig. 3: Analysis of the chemical bonding in the hexagonal planar geometry.

Data availability

Crystallographic models are available as .cif files from the Cambridge Crystallographic Data Centre (CCDC, https://www.ccdc.cam.ac.uk); CCDC numbers 1589323–1589324, 1909687–1909690 and 1946045). Neutron diffraction images can be obtained from A.J.E. The derived structure factors have been deposited with the CCDC, with CCDC number 1946045. Data associated with DFT calculations (.xyz coordinate file) along with NMR spectroscopic data (.mnova files) are available from a public repository at https://doi.org/10.14469/hpc/5985. Full details of the syntheses are provided in the Supplementary Information.

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Acknowledgements

We thank the Royal Society and the European Research Council (FluoroFix: 677367) for funding, Johnson Matthey for the gift of PdCl2, and ANSTO for allocation of neutron beam time on KOALA to proposal P6932. We thank Oxford Cryosystems Ltd and ANSTO for funding a Russell Studentship with the University of Oxford (GAS).

Author information

M.G. and C.B. carried out the synthetic studies. M.G. conducted DFT, QTAIM and related calculations. A.J.P.W. collected, processed and refined single-crystal X-ray diffraction data. A.J.E., R.I.C. and G.A.S. undertook the single-crystal Laue neutron diffraction experiment, data reduction and refined the neutron diffraction structural model. M.R.C. managed the project. All authors contributed to the writing and editing of the manuscript.

Correspondence to Mark R. Crimmin.

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The authors declare no competing interests.

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Peer review information Nature thanks Jean-François Halet and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Extended data figures and tables

Extended Data Fig. 1 Synthesis of group-10 hydride complexes with magnesium ligands.

a–e, Synthetic schemes for the preparation of complexes 1a (a), 2a (b), 2b (c), 2c (d) and 3 (e); for 3, the hydride ligands are derived from the C–H bonds of the benzene solvent.

Supplementary information

Supplementary Information

This file contains Supplementary Figures S1–S28 and Supplementary Tables S1–S12.

Supplementary Data

This cif file contains Neutron Diffraction Data.

Supplementary Data

This cif file contains X-ray Diffraction Data.

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