Relativistic effects are crucial for uranium-phosphorus bonding: a DFT study of cyclopentadienyluranium-phosphine complexes

The influence of relativistic effects on the uranium–phosphorus bond in different cyclopentadienyluranium–phosphine complexes was systematically investigated using density functional theory (DFT). Calculations utilising the zeroth-order regular approximation (ZORA) and effective core potentials (ECPs) with varying levels of relativistic treatment (Dirac-Hartree-Fock, Wood-Boring, and non-relativistic) were compared. Structural parameters, bonding characteristics (via FMO, NBO, and QTAIM analyses), and bond dissociation energies were evaluated. While relativistic ECPs and ZORA reproduce experimental bond lengths and bonding character, non-relativistic calculations overestimate U–P bonds by more than 5% and underestimate covalency, with bond dissociation energies reduced by up to 47 kJ/mol. Non-relativistic calculations revealed a better energy match of the frontier orbitals, but these are outweighed by the improved orbital overlap when including relativistic effects. Our results demonstrate that relativistic effects are crucial for the accurate description of actinoid-main group element bonds and highlight the necessity of including relativistic corrections for quantitative computational studies of heavy element chemistry.