The geometries of transition-metal complexes are a small clique. The atoms and ligands in such complexes generally take on only a few structures, such as octahedral, trigonal bipyramidal, and square planar shapes. Scientists proposed a hexagonal planar geometry more than 100 years ago, but it has never been captured in crystal form until now. Mark R. Crimmin and coworkers at Imperial College London nabbed the structure with a complex consisting of a palladium atom surrounded by three hydride and three magnesium-diisopropylphenyl ligands (Nature 2019, DOI: 10.1038/s41586-019-1616-2). Scientists know that hexagonal planar compounds can form in condensed metal phases and in the pores of coordination polymers, but a transition-metal complex with this geometry was a surprise, Crimmin says. “The magnesium ligands accept electron density from the palladium,” and, with this geometry, they interact optimally with palladium's filled d orbitals to stabilize the compound, he adds. “Not only does this species have the potential for interesting reactivity,” says Michael Whittlesey of the University of Bath, but it suggests that combining transition metals with other nonconventional main-group metals may give rise to compounds with unexpected properties.
This story was updated on Oct. 15, 2019, to correct the statement that the hydrides overlap well with filled palladium d orbitals. The magnesium ligands are what interact with palladium's d orbitals.