Quantifying the Interdependence of Metal–Ligand Covalency and Bond Distance Using Ligand K‐edge XAS

Lee, Kyounghoon; Blake, Anastasia V.; Donahue, Courtney M.; Spielvogel, Kyle D.; Bellott, Brian J.; Daly, Scott R.

doi:10.1002/anie.201905635

Cited by 12 publications

(9 citation statements)

References 39 publications

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“…The shorter than expected bridging U−B distances suggest that the U−H−B bonds are more covalent than the Ln−H−B bonds. Consistent with this analysis, we have shown that bond length differences as small as 0.04 Å can give rise to measurable differences in metal‐ligand covalency using ligand K‐edge XAS [22] . Moreover, the 0.04 Å difference is comparable to the 0.036 Å difference reported recently by Goodwin et al.…”

Section: Methodssupporting

confidence: 91%

Quantifying the Influence of Covalent Metal‐Ligand Bonding on Differing Reactivity of Trivalent Uranium and Lanthanide Complexes

et al. 2022

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Qualitative differences in the reactivity of trivalent lanthanide and actinide complexes have long been attributed to differences in covalent metal-ligand bonding, but there are few examples where thermodynamic aspects of this relationship have been quantified, especially with U 3 + and in the absence of competing variables. Here we report a series of dimeric phosphinodiboranate complexes with trivalent f-metals that show how shorter-than-expected UÀ B distances indicative of increased covalency give rise to measurable differences in solution deoligomerization reactivity when compared to isostructural complexes with similarly sized lanthanides. These results, which are in excellent agreement with supporting DFT and QTAIM calculations, afford rare experimental evidence concerning the measured effect of variations in metal-ligand covalency on the reactivity of trivalent uranium and lanthanide complexes.One of the most critical debates in f-element science continues to center on the role of covalent metal-ligand bonding on the reactivity of trivalent lanthanide and actinide complexes. [1] As proposed by Seaborg and co-workers in the 1950's, [2] suspected differences in covalent metal-ligand bonding with 4f and 5f metals are often invoked when attempting to account for reactivity differences observed with trivalent actinides and lanthanides, [3] especially in processes related to separations and ligand binding studies. [4] Prominent examples reported by Jensen, Nash, and coworkers, for example, ascribed Am 3 + /Ln 3 + binding enthalpy[*] T.

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Section: Methodssupporting

confidence: 91%

Quantifying the Influence of Covalent Metal‐Ligand Bonding on Differing Reactivity of Trivalent Uranium and Lanthanide Complexes

et al. 2022

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“…Consistent with this analysis, we have shown that bond length differences as small as 0.04 Å can give rise to measurable differences in metal-ligand covalency using ligand K-edge XAS. [22] Moreover, the 0.04 Å difference is comparable to the 0.036 Å difference reported recently by Goodwin et al for more covalent UÀ Se bonds compared to LaÀ Se in M[N-(Se=PPh 2 ) 2 ] 3 complexes. [23] Given that the bridging MÀ HÀ B bonds are the bonds that must be broken for the M 2 (H 3 BP t Bu 2 BH 3 ) 6 dimers to convert to monomers, we suspected that the thermochemical values associated with this process should reflect the enhanced covalency in the UÀ HÀ B bonds.…”

supporting

confidence: 76%

Quantifying the Influence of Covalent Metal‐Ligand Bonding on Differing Reactivity of Trivalent Uranium and Lanthanide Complexes

et al. 2022

Self Cite

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Qualitative differences in the reactivity of trivalent lanthanide and actinide complexes have long been attributed to differences in covalent metal‐ligand bonding, but there are few examples where thermodynamic aspects of this relationship have been quantified, especially with U3+ and in the absence of competing variables. Here we report a series of dimeric phosphinodiboranate complexes with trivalent f‐metals that show how shorter‐than‐expected U−B distances indicative of increased covalency give rise to measurable differences in solution deoligomerization reactivity when compared to isostructural complexes with similarly sized lanthanides. These results, which are in excellent agreement with supporting DFT and QTAIM calculations, afford rare experimental evidence concerning the measured effect of variations in metal‐ligand covalency on the reactivity of trivalent uranium and lanthanide complexes.

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“…Spectra at the P and Cl K-edges were measured initially to probe electronic structure from the perspective of heavier atoms in the coordination sphere. Previous studies have shown that P or Cl K-edge XAS can be sensitive to bonding with phosphine or chloride ligands directly bound to metal centers. − ,− Figure shows the background-subtracted and normalized P K-edge XAS spectra for the free ligands TRPO and their uranyl complexes UO 2 Cl 2 (TRPO) 2 . In the P K-edge XAS of the TRPO series, the pre-edge features show substantial differences with the variation of the R substituents.…”

Section: Resultsmentioning

confidence: 99%

Involvement of 5f Orbitals in the Covalent Bonding between the Uranyl Ion and Trialkyl Phosphine Oxide: Unraveled by Oxygen K-Edge X-ray Absorption Spectroscopy and Density Functional Theory

et al. 2021

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Monodentate organophosphorus ligands have been used for the extraction of the uranyl ion (UO 2 2+) for over half a century and have exhibited exceptional extractability and selectivity toward the uranyl ion due to the presence of the phosphoryl group (OP). Tributyl phosphate (TBP) is the extractant of the world-renowned PUREX process, which selectively recovers uranium from spent nuclear fuel. Trialkyl phosphine oxide (TRPO) shows extractability toward the uranyl ion that far exceeds that for other metal ions, and it has been used in the TRPO process. To date, however, the mechanism of the high affinity of the phosphoryl group for UO 2 2+ remains elusive. We herein investigate the bonding covalency in a series of complexes of UO 2 2+ with TRPO by oxygen K-edge X-ray absorption spectroscopy (XAS) in combination with density functional theory (DFT) calculations. Four TRPO ligands with different R substituents are examined in this work, for which both the ligands and their uranyl complexes are crystallized and investigated. The study of the electronic structure of the TRPO ligands reveals that the two TRPO molecules, irrespective of their substituents, can engage in σand π-type interactions with U 5f and 6d orbitals in the UO 2 Cl 2 (TRPO) 2 complexes. Although both the axial (O yl ) and equatorial (O eq ) oxygen atoms in the UO 2 Cl 2 (TRPO) 2 complexes contribute to the Xray absorption, the first pre-edge feature in the O K-edge XAS with a small intensity is exclusively contributed by O eq and is assigned to the transition from O eq 1s orbitals to the unoccupied molecular orbitals of 1b 1u + 1b 2u + 1b 3u symmetries resulting from the σand π-type mixing between U 5f and O eq 2p orbitals. The small intensity in the experimental spectra is consistent with the small amount of O eq 2p character in these orbitals for the four UO 2 Cl 2 (TRPO) 2 complexes as obtained by Mulliken population analysis. The DFT calculations demonstrate that the U 6d orbitals are also involved in the U−TRPO bonding interactions in the UO 2 Cl 2 (TRPO) 2 complexes. The covalent bonding interactions between TRPO and UO 2 2+ , especially the contributions from U 5f orbitals, while appearing to be small, are sufficiently responsible for the exceptional extractability and selectivity of monodentate organophosphorus ligands for the uranyl ion. Our results provide valuable insight into the fundamental actinide chemistry and are expected to directly guide actinide separation schemes needed for the development of advanced nuclear fuel cycle technologies.

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Quantifying the Interdependence of Metal–Ligand Covalency and Bond Distance Using Ligand K‐edge XAS

Cited by 12 publications

References 39 publications

Quantifying the Influence of Covalent Metal‐Ligand Bonding on Differing Reactivity of Trivalent Uranium and Lanthanide Complexes

Quantifying the Influence of Covalent Metal‐Ligand Bonding on Differing Reactivity of Trivalent Uranium and Lanthanide Complexes

Quantifying the Influence of Covalent Metal‐Ligand Bonding on Differing Reactivity of Trivalent Uranium and Lanthanide Complexes

Involvement of 5f Orbitals in the Covalent Bonding between the Uranyl Ion and Trialkyl Phosphine Oxide: Unraveled by Oxygen K-Edge X-ray Absorption Spectroscopy and Density Functional Theory

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