4f-orbital covalency enables a single-crystal-to-single-crystal ring-opening isomerization in a Ce-cyclopropenyl complex.

Metal-ligand bonding interactions for f-element compounds are typically highly polarized with only minor covalent character. Whereas the 5d/6d orbitals are known to be chemically accessible for dative bonding, recent quantum chemical and spectroscopic analyses have indicated appreciable 4f/5f-orbital involvement in certain metal-ligand bonds. However, 4f-orbital covalency has not been compellingly linked to distinctive modes of chemical reactivity via rigorous comparative study and mechanistic investigation. Here a series of M-cyclopropenyl complexes (M = Ti, Zr, Ce, Hf, Th) are described, wherein the cerium congener exhibits a 4f-covalent Ce=C interaction, causing a ring-opening isomerization reaction through a single-crystal-to-single-crystal transformation. The results provide evidence for 4f-orbital covalency by demonstrating its expression in the reactivity of an f-element complex within an isostructural series of tetravalent d- and f-block metal complexes. They also provide new directions for the study of orbital covalency effects of molecular compounds in solid-state chemical transformations.