In this month’s ‘in your element’ article (subscription required), Eric Schelter from the University of Pennsylvania draws our attention to cerium, an element that serves a variety of commercial and industrial applications, yet presents chemists with some rather peculiar challenges.
Although it is one of the rare-earth elements, cerium is fairly abundant in the Earth’s crust, and widely used for various purposes. The oxide ceria (CeO2), for example, is a common abrasive for the polishing of surfaces ranging from optical lenses to electronic displays. The reason it is particularly efficient is that, in addition to a mechanical polishing action, it attacks the basic sites of surfaces.
Most of cerium’s applications rely on its interconversion between the +3 and +4 oxidation states. I’ll let you read in the article how “hydrocarbon fuels encounter element 58 at both the beginning and the end of their useful life”, and which cerium compound represents a “drastic ‘nuclear option’ for oxidation reactions” in synthetic chemistry.
There is much to explore regarding the reactivity of cerium, and even in terms of the electronic structure of some of its compounds — this is an aspect that I find particularly intriguing. Take cerocene, a seemingly straightforward complex that consists of a cerium centre sandwiched between two C8H8 ligands to form an eclipsed sandwich complex. Experimental characterization and computational calculations point to a multiconfigurational ground state, for which it’s proving rather difficult to determine unambiguously the Ce(III) and Ce(IV) contributions. As Schelter puts it, “this deceptively simple compound represents a stimulating case where the very human concept of a formal oxidation state fails to capture the essential essence of a molecule.”
Anne Pichon (Senior Editor, Nature Chemistry)