A Spectroscopic and Computational Evaluation of Uranyl Oxo Engagement with Transition Metal Cations

We report the synthesis and characterization of five novel Cd²⁺/UO₂²⁺ heterometallic complexes that feature Cd-oxo distances ranging from 78 to 171% of the sum of the van der Waals radii for these atoms. This work marks an extension of our previously reported Pb²⁺/UO₂²⁺ and Ag⁺/UO₂²⁺ complexes, yet with much more pronounced structural and spectroscopic effects resulting from Cd-oxo interactions. We observe a major shift in the U═O symmetric stretch and significant uranyl bond length asymmetry. The ρ_bcp values calculated using Quantum Theory of Atoms in Molecules (QTAIM) support the asymmetry displayed in the structural data and indicate a decrease in covalent character in U═O bonds with close Cd-oxo contacts, more so than in related compounds containing Pb²⁺ and Ag⁺. Second-order perturbation theory (SOPT) analysis reveals that O sp → Cd s is the most significant orbital overlap and U═O bonding and antibonding orbitals also contribute to the interaction (U═O σ/π → Cd d and Cd s → U═O σ/π*). The overall stabilization energies for these interactions were lower than those in previously reported Pb²⁺ cations, yet larger than related Ag⁺ compounds. Analysis of the equatorial coordination sphere of the Cd²⁺/UO₂²⁺ compounds (along with Pb²⁺/UO₂²⁺ complexes) reveals that 7-coordinate uranium favors closer, stronger M-oxo contacts. These results indicate that U═O bond strength tuning is possible with judicious choice of metal cations for oxo interactions and equatorial ligand coordination.