Using NMR Spectroscopy to Evaluate Metal-Ligand Bond Covalency for the f Elements

Area of Science:

• Inorganic Chemistry
• Physical Chemistry
• Nuclear Chemistry

Background:

• Understanding f-element ligand covalency is crucial for designing separations schemes for spent nuclear fuel.
• Existing methods like XANES, EPR, optical spectroscopies, and X-ray crystallography have limitations in measuring 4f/5f covalency.
• Multinuclear NMR spectroscopy offers a complementary approach to probe metal-ligand bonding in f-element systems.

Purpose of the Study:

• To develop and validate multinuclear NMR spectroscopy for quantifying metal-ligand covalency in actinides and lanthanide complexes.
• To establish NMR spectroscopy as a reliable tool for analyzing electronic structure in f-element chemistry.

Main Methods:

• Utilized multinuclear NMR spectroscopy to study metal-ligand covalency across various f-element complexes.
• Quantified covalency using chemical shift (δ) and nuclear shielding constants (σ) for isotopes like ¹³C, ¹⁵N, ⁷⁷Se, and ¹²⁵Te.
• Employed relativistic density functional theory (DFT) to analyze diamagnetic, paramagnetic, and spin-orbit contributions to nuclear shielding, focusing on the spin-orbit contribution (Δ_SO).

Main Results:

• Successfully quantified M-L covalency for diverse ligand types (chalcogenides, carbenes, alkyls, amides, nitrides) and isotopes.
• Demonstrated that the spin-orbit contribution (Δ_SO) effectively correlates with the degree of covalency, with large values for highly covalent bonds (e.g., [U(CH₂SiMe₃)₆]) and small values for ionic bonds (e.g., [La(C₆Cl₅)₄]^-).
• Showcased the exquisite sensitivity of NMR spectroscopy to unravel electronic structure, even for subtle bonding effects, requiring inclusion of spin-orbit effects for accurate predictions.

Conclusions:

• Multinuclear NMR spectroscopy, particularly when incorporating spin-orbit effects in DFT calculations, is a powerful and sensitive technique for probing f-element ligand covalency.
• This method provides valuable insights into electronic structure, complementing established techniques and advancing the fundamental understanding of f-element bonding.
• The findings support the development of advanced separation technologies for nuclear fuel by offering a precise tool to characterize f-element interactions.