Covalency in Actinide Compounds.

Covalency in actinides has emerged as a resounding research topic on account of the technological importance in separating minor actinides from lanthanides for spent nuclear fuel processing, and utilization of their distinct bonding properties has been realized as a route towards overcoming this challenge. Because of the  limited radial extent of the 4 f orbitals , there is almost no 4 f  electron participation in bonding in lanthanides; this is not the case for the actinides, which have extended 5 f  orbitals that are capable of overlapping with ligand orbitals, although not to the degree of overlap as in the  d  orbitals of transition metals. In this concept paper, we provide a general description of covalency in actinide compounds. After introducing two main approaches to enhance covalency, either by exploiting increased orbital overlap or decreasing energy differences between the orbitals causing orbital energy degeneracy, we show the current state of the field using several examples from the recent literature. We will conclude by proposing the use of actinide chalcogenides as a convenient auxiliary tool to study covalency in actinide compounds.