Covalent vs electrostatic interactions in rare earth systems: a comparative study of U(III), U(IV), and U(V) and Nd(III) bonding properties by DFT and CAS-PT2 approaches.

A description of the electronic structure of F(3)UCO, F(3)NdCO, F(4)UCO, and F(5)UCO has been obtained by Complete Active Space second-order perturbation theory CASPT2 calculations using a relativistic effective core potential. These multiconfigurational calculations have been compared to the DFT description combined with a quasi-relativistic ZORA scalar approach. Geometries have been optimized for both levels of calculations and frequencies computed in the DFT formalism. The bonding properties of U(III) have been compared to those of Nd(III) and of higher oxidation states of U(IV,V). Both methodologies are consistent and show a decrease of the covalent character of the U-CO bonding with a higher oxidation state, U(IV) or U(V), as well as its absence for for the isoelectronic Nd(III) species.