Hexagonal Planar Complexes: Structure, Bonding, and Catalytic Relevance

Despite over a century of advances in the field of coordination chemistry, transition metal complexes remain limited to a handful of well understood geometries. Archetypal geometries for six-coordinate transition metals are octahedral and trigonal prismatic. Although deviations from ideal bond angles and lengths are common, alternative parent geometries are staggeringly rare. Hexagonal planar transition metals are restricted to those found in condensed metallic phases, the hexagonal pores of coordination polymers, or clusters containing more than one transition metal in proximity.

In this lecture, I will discuss the coordination of molecular zinc and magnesium hydrides to transition metals. I will define a continuum of bonding between hexagonal planar and trigonal planar geometries and explore examples of complexes that exist along this continuum. Aspects of bonding and electronic structure will be covered. I will conclude by describing the relevance of these complexes to proposed on-cycle intermediates in catalytic C–H functionalisation of aromatics and heteroaromatics and H/D-exchange reactions.