Since the original elaboration of the peculiar spectroscopic and magnetic properties of cerocene [Ce(COT)₂, COT = cyclooctatetraene], complexes featuring Ce^IV–C chemical bonds have gathered increasing interest among the scientific community. Metal-ligand bonding interactions in lanthanides coordination compounds have, until recently, been regarded as primarily ionic with only minor covalent contributions from metal-based 5d-orbitals in certain instances. This model has been revised in recent decades as reports emerged of lanthanide complexes showing spectroscopic evidence of 4f-orbital covalency. Many of the reports of 4f-covalency in Ce^IV coordination compounds rely on the combined analysis of quantum chemical calculations, NMR spectroscopy, and X-ray absorption near edge spectroscopy (XANES) to support the claims of 4f-orbital covalency. NMR analyses of coordinating nuclei provide chemical shifts that report on the relative metal orbital contributions to bonds, while XANES spectroscopy can inform on changes in f-occupancy, a measure of the electron density in the 4f subshell, that result from changes in ligand donation bonding with the 4f orbitals.
Despite the progress in organocerium (IV) chemistry in recent decades, 4f-orbital covalency has not been compellingly linked to novel modes of chemical reactivity through rigorous comparative study. To that end, we synthesized a series of M^IV-cyclopropenyl complexes (M = Ti, Zr, Ce, Hf, Th) and discovered that the cerium congener exhibits a 4f-covalent Ce=C_α interaction, causing a unique ring-opening isomerization reaction through a single-crystal-to-single-crystal (SCSC) transformation that was tracked using single-crystal diffractometry. Measurement of the reaction rates of a series of single crystals of the Ce^IV-cyclopropenyl complex of variable volumes under isothermal conditions yielded an apparently linear relationship between crystal volume and the reaction rate. Using an identical approach with single crystals of the deuterium labelled isotopologue of the Ce^IV-cyclopropenyl complex revealed an inverse kinetic isotope effect (IKIE) for the reaction. The results are consistent with a reversible ring-opening step to form a Ce^IV-vinylcarbene intermediate followed by an irreversible 1,2-H shift to yield the Ce^IV-allenyl product and provide compelling experimental support for the computed mechanism of the reaction.

Patrick Smith

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