Metal–metal bonding in the actinide elements: Conceptual synthesis of a pure two-electron U–U fδ single bond in a constrained geometry of U2(OH)10

•The formation of a molecule with a pure U–U 5fδ bond is conceptualized.•A constrained geometry of U2(OH)10 is proposed to satisfy the requirements to form the 5fδ bond.•Preliminary DFT calculations indicate that both the 5fs and 5fδ bonding orbitals in U2(OH)10 are close in energy, leading to a more complex electronic structure than hoped.

By utilizing high symmetry and geometric constraints, it is proposed that a molecule with a pure U–U 5fδ bond can be constructed. The proposed molecule is D4h U2(OH)10 with the following geometric constraints: (1) All of the U–O–H linkages are linear, assuring that each linear hydroxide group has two filled π orbitals capable of donating to the U atoms, and (2) all of the U–U–O(eq) bond angles are 90°, which assures that all of the U valence orbitals except for the fδ[zxy] orbital are destabilized by ligand donation. Preliminary DFT calculations with relativistic effective core potentials indicate that the fσ orbital is not destabilized enough in this simple model, and that it is essentially degenerate with the fδ orbital, likely leading to a triplet (fσ)1(fδ)1 single bond. Single point calculations were used to estimate the U–U bond length and bond strength of the U–U single bond in constrained U2(OH)10 with an (fσ)2(fδ)0 configuration. We estimate that the bond length is 3.55 Å and the bond strength is 9.7 kcal/mol for this very fragile molecule.

Graphical abstractA conceptual modeling of a ligand architecture is discussed that could lead to a uranium–uranium single bond that is a pure 5fδ bond. The initial molecule that could possibly lead to this type of bonding is a constrained geometry of the U(V)–U(V) dimer U2(OH)10. Preliminary DFT calculations with relativistic corrections on this molecule indicate that it is close to achieving to the unusual desired bonding.Figure optionsDownload full-size imageDownload as PowerPoint slide