A hexagonal planar transition-metal complex

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 theory^2–5. 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 proximity^10,11. Such a geometry had been considered^12,13 for [Ni(P^tBu)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.