Chemists at the University of Manchester, working with Lancaster University, have isolated uranocenium, the positively charged form of the uranium sandwich compound uranocene. Uranocene itself was first made in 1968, but its cation eluded isolation for more than half a century. The team discovered that earlier attempts failed not because the molecule was unstable, but because a fluorine-containing electrolyte salt used in those experiments reacted with and destroyed the cation.
By switching to a different oxidising agent and a less reactive solvent, the researchers obtained crystals that are stable at room temperature. They characterised the compound with single-crystal X-ray diffraction, magnetic measurements, electron paramagnetic resonance, electrochemistry and computational modelling. The results, published in Nature Communications, mean chemists now have uranium sandwich molecules in three different oxidation states, a rare toolkit for probing bonding in the actinide series.
The structural details carry the real surprise. Although uranium moves from the +4 to the +5 oxidation state on losing an electron, the uranium-carbon distances shortened by only about 0.02 angstroms, several times less than expected. The explanation is increased covalent electron sharing: uranium's share of the key bonding electrons rose from around 30 per cent to more than 40 per cent, evidence that covalency, often assumed marginal in actinide chemistry, can actively hold a molecule's architecture together.