Uranyl complexation by monodentate nitrogen donor ligands. A relativistic density functional study

To examine the interaction of uranyl with nitrogen containing groups of humic substances, the model complexes [UO₂(H₂O) ₄L_N]²⁺, L_N = NH₂CH ₃, N(CH₃)₃, and NC₅H₅ in aqueous solution were studied computationally with an all-electron relativistic density functional method. Results are compared with the corresponding penta-aqua complex of uranyl. Although pyridine coordinates with about the same strength as L = H₂O, methylamine binds ∼10 kJ mol^-1 stronger and trimethylamine ∼40 kJ mol^-1 weaker than a fifth aqua ligand. Yet, each of these ligands L_N donates about the same amount of charge to uranyl as L = H₂O. U-N bonds are ∼10 pm longer than the U-O bonds of the aqua ligands. From the present model results, one does not expect that, when compared with carboxyl groups, monodentate N-containing functional groups contribute significantly to uranyl complexation by humic substances.