On the Origin of Covalent Bonding in Heavy Actinides

Abstract

Recent reports have suggested the late actinides participate in more covalent interactions than the earlier actinides, yet the origin of this shift in chemistry is not understood. This report considers the chemistry of actinide dipicolinate complexes to identify why covalent interactions become more prominent for heavy actinides. A modest increase in measured actinide:dipicolinate stability constants is coincident with a significant increase in An 5f energy degeneracy with the dipicolinate molecular orbitals for Bk and Cf relative to Am and Cm. While the interactions in the actinide-dipicolinate complex are largely ionic, the decrease in 5f orbital energy across the series manifests in orbital-mixing and, hence, covalency driven by energy degeneracy. This observation suggests the origin of covalency in heavy actinide interactions stems from the degeneracy of 5f orbitals with ligand molecular orbitals rather than spatial orbital overlap. These findings suggest that the limiting radial extension of the 5f orbitals later in the actinide series could make the heavy actinides ideal elements to probe and tune effects of energy degeneracy driven covalency.

δ and φ back-donation in AnIV metallacycles

“…Hence, PBE data was used throughout the manuscript. This is also consistent with the previous success by the PBE functional in studying complicated actinide compounds [62][63][64] . Since in general GGA functionals like PBE tend to over-delocalize electrons, potentially magnifying the effect of the δ and φ bonds discussed here, we also employed PBE0 hybrid functional.…”

Charge densities in actinide compounds: strategies for data reduction and model building

“…Participation of the 5f or 6d orbitals (or both) in covalent bonding has been considered for many years, although the amount of covalent character in the highly polar actinide-element bond is still a subject of debate. In contrast to the chemically familiar orbital overlap concept of covalency in light elements, covalency in actinides may be enhanced by small energy differences between metal and ligand orbitals (Su et al, 2018;Ingram et al, 2008;Tanti et al, 2018;Gregson et al, 2016;Kerridge, 2017;Kaltsoyannis, 2013Kaltsoyannis, , 2016Kaltsoyannis, , 2018Jones et al, 2013;Kirker & Kaltsoyannis, 2011;Prodan et al, 2007;Neidig et al, 2013;Kelley et al, 2017). This observation has provoked further debate as to whether 'genuine' covalency ought to increase or decrease across the actinide series.…”

Spectroscopic and Computational Characterization of Diethylenetriaminepentaacetic Acid/Transplutonium Chelates: Evidencing Heterogeneity in the Heavy Actinide(III) Series

“…In contrast to the lanthanide (Ln) series, transplutonium chemistry studies are scarce, and most of the data have been acquired using tracer techniques or computational approaches . Among the trivalent actinides (An), Cm is undoubtedly the most studied, owing to its luminescence properties and the availability of its long‐lived isotope 248 Cm ( t 1/2 =3.49×10 5 years) to a handful of research institutions.…”