In Search of Covalency Measure of Gd(III)-Ligand Interactions

Experimental electron density distribution of the [C(NH₂)₃]₃[Gd(EDTA)F₂]·H₂O crystal was determined. The derived experimental and theoretical (DFT) topological parameters such as ∇²ρ_c, ρ_c, bond degree (BD), kinetics, and potential energy were used to study the nature of Gd-O, Gd-F, and Gd-N interactions. The natural charge of the Gd is 1.86; the natural configuration of the cation is [Xe]6s^0.134f^7.105d^0.83, and the covalency of the Gd-L bond is mainly connected with the transfer of charge from the sp^<i>x</i> ligand orbitals onto the 5d orbitals of the Gd cation. Simultaneously, the donation of charge onto the 6s and 4f orbitals occurs to a lesser extent. Moreover it was found that the donation of the ligand charges onto the Gd(III) is larger for compounds with a lower coordination number. The obtained topological parameters were analyzed in the context of the Gd(III) f-f transition properties, i.e., energy of the excited ^2<i>S</i>+1L_J states, Judd-Ofelt intensity parameters, and luminescence lifetimes, of 18 Gd(III) compounds with various O, N, and F donor ligands (DOTA, EDTA, CDTA, DTPA, NTA, EGTA, ODA, F^-, H₂O, and CO₃^2-). The calculated nephelauxetic β parameter may reflect the penetration degree of electron lone pairs of ligands inside the metal basin. Finally, it was found for the first time that the sum of the Gd(III)-L bond energy (∑<i>E</i>_GdL) is correlated with the position of the gravity center of the ⁸S_7/2 → ^2<i>S</i>+1L_J transitions and increase of covalency of the Gd(III)-L bonds is associated with decrease of their bond energy. The obtained results may shed light on chemical bonding in systems containing f-elements. Such subtle differences in the covalent contribution to the Ln-L or An-L bond may tune the selectivity of the partitioning processes of lanthanides and actinides.