Theoretical Investigations of Uranyl−Ligand Bonding:  Four- and Five-Coordinate Uranyl Cyanide, Isocyanide, Carbonyl, and Hydroxide Complexes

Abstract

The coordination and bonding of equatorial hydroxide, carbonyl, cyanide (CN-), and isocyanide (NC-) ligands with uranyl dication, [UO2]2+, has been studied using density functional theory with relativistic effective core potentials. Good agreement is seen between experimental and calculated geometries of [UO2(OH)4]2-. Newly predicted ground-state structures of [UO2(OH)5]3-, [UO2(CO)4]2+, [UO2(CO)5]2+, [UO2(CN)4]2-, [UO2(CN)5]3-, [UO2(NC)4]2-, and [UO2(NC)5]3- are reported. Four-coordinate uranyl isocyanide complexes are the predicted gas-phase species while five-coordinate uranyl cyanide complexes are energetically favorable in aqueous solution. Small energy differences between cyanide and isocyanide complexes indicate the energetic feasibility of mixed cyanide and isocyanide complexes. A D2d uranyl tetrahydroxide is the dominant gas-phase and aqueous species, but formation of uranyl carbonyl complexes is seen to be exothermic in the gas-phase and endothermic in aqueous solution.

A QM/MM study on the aqueous solvation of the tetrahydroxouranylate [UO2(OH)4]2− complex ion

“…Clark's interpretation is refuted by Wahgren, Grenthe, Vallet, and coworkers, 16 who argue that it is more likely that the four hydroxo complex that is observed in crystal structures, also prevails in solution. This interpretation is supported by recent theoretical work of Sonnenberg et al, 15 who use a continuum model to include solvent effects and find that coordination of a fifth hydroxo ligand is endothermic by 107 kJ/mol. In our earlier work we showed that in the case of complexation with fluoride anions reduction of the electrostatic repulsion of the fluoride ligands by charge transfer to the surrounding water molecules is important.…”

Density Functional Studies on the Complexation and Spectroscopy of Uranyl Ligated with Acetonitrile and Acetone Derivatives

“…Additionally, the sixth benzonitrile addition is predicted to be endothermic by 0.8 kcal/mol. This value is quite small and does not necessarily prohibit the formation of [UO 2 (Bzn) 6 ] 2+ , considering the accuracy of the energetics coming from the DFT methodology used in this work. The process by which these complexes are formed in the experiments is energetic, and considering the weak binding energy, it is not surprising that these two complexes were not observed.…”

“…With five acetones, the plane of each ligand is rotated 7-26°f rom the equatorial plane ( Figure 6). The acetones in [UO 2 (Ace) 6 ] 2+ cannot all lie in the equatorial plane ( Figure 7). The oxygens deviate from this plane by 9°and alternate above and below the equatorial plane.…”

Investigation of pure and Co2+-doped ZnO quantum dot electronic structures using the density functional theory: choosing the right functional

“…Historically, bulk semiconductors have often been modeled using a plane wave basis within the local density approximation (LDA) or generalized gradient approximation (GGA) of the DFT. Meanwhile, the energetics and physical properties of molecular transition metal complexes are better described with either meta-GGA [17] or hybrid DFT functionals [17,18], in which a certain amount of Fock exchange is added to part of the local or semilocal density functional exchange energy. Hybrid functionals might be anticipated to perform better than LDA or pure DFT functionals, but the accuracies of various DFT functionals for studies of DMS nanocrystals have not yet been systematically investigated.…”