Abstract and subjects
► Extensive experimental evidence exists for covalency in f-element complexes. ► 5f and 6d as well as pseudo-core 6s and 6p orbitals can contribute to bonding in light actinides. ► Both overlap and near-degeneracy driven covalency may be operative for different classes of complexes.
The presence of covalency in complexes of the 4f and 5f elements has been a source of intense research and controversy. In addition to academic interest in this debate, there is an industrial motivation for better understanding of bonding in f-element complexes due to the need to separate trivalent trans-plutonium elements from trivalent lanthanide fission products in advanced nuclear fuel cycles. This review discusses the key evidence for covalency in f-element bonds derived from structural, spectroscopic and theoretical studies of some selected classes of molecules, including octahedral hexahalides, linear actinyl and organometallic sandwich complexes. This evidence is supplemented by a discussion of covalency, including the possibility of both overlap and near-degeneracy driven covalency and the need to quantify their relative contributions in actinide metal–ligand bonds.