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 theory2-5. 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.