Exploring Covalency in f-Element Complexes: Dithiocarbamate Ligands Reveal Differences Between Heavy Actinides and Lanthanides

Abstract

Covalence in f-elements plays a pivotal role in distinguishing the fundamental properties of actinides and lanthanides. In this study, trivalent and tetravalent actinide (U–Fm) and lanthanide (Nd–Er) complexes with dithiocarbamate (S2CNH2–) ligands were systematically analyzed. Various methodologies were employed, notably Slater–Condon parameters, metrics in bond critical points (BCP) under the quantum theory of atoms in molecules (QTAIM) framework, and natural localized molecular orbitals (NLMOs). The multiconfigurational nature of the systems and the scalar relativistic and spin–orbit coupling effects were incorporated into the methods used. The findings reveal important differences in covalence between heavy actinides and lanthanides, where a higher covalence is observed in complexes containing metal ions with a higher oxidation state. According to QTAIM results, covalence in heavy actinides is energy-degeneracy driven rather than orbital overlap. Additionally, this suggests that donor atoms softer than oxygen, such as sulfur, promote covalence in heavy actinides, distinguishing them from their lanthanide counterparts and establishing them as softer Lewis acids.