Low-valent Ln(II) and An(II) have been isolated and characterized in solid-state complexes. Many of these M(II) centers are stabilized in a pseudo C3V coordination environment by derivatized ancillary Cp ligands, as demonstrated in the series of complexes [MIICp3]1- (M = Ln/U, Cp = C5H4SiMe3 (Cp’) or 1,3-(SiMe3)2C5H3 (Cp”)) and [AnIICp”3]1- (An = Th, U, Pu, Np).1 . The tris-Cp ligand field has been shown to have unique effects on the electronic configuration of the metal center. For example, Ln2+ species supported by tris-Cp’ environments were shown to exhibit the unusual electron configuration 4fn5d1 (rather than the 4fn+15d0 configuration of conventional divalent lanthanides). Distinct d-orbital occupation also occurs in [AnIICp”3]1- anions of Th2+ (5f06d2) and U2+ (5f36d1). In the transuranic realm, descriptions become less straightforward, with DFT modeling of the structurally characterized Np2+ and Pu2+ Cp” complexes suggesting configuration crossover between elements 93 and 94 (i.e., switching between 5fn6d1 and 5fn+16d0 as the most stable ground state).2 Investigating factors that control and tune the electronic structure is central to forming descriptions of this crossover behavior. Systematic electronic structure studies have been performed by varying the identity and properties of coordinating ligands across the An2+ series. The physical and electronic properties of these homologous series of molecular complexes has been evaluated by UV-Vis, NMR, EPR, EXAFs, and XANES.

Elucidation of electronic configuration and bonding in organometallic actinide complexes

ACS Fall 2021

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