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

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.