A spectroscopic and computational evaluation of uranyl oxo engagement with transition metal cations

We report the synthesis and characterization of five novel Cd2+/UO¬22+ 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 ρbcp¬ 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 spx 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+/UO¬22+ compounds (along with Pb2+/UO¬22+ complexes) reveals that 7-coodinate uranium favors closer, stronger Mn+-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.