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 C_3V coordination environment by derivatized ancillary Cp ligands, as demonstrated in the series of complexes [M^IICp₃]^1-(M = Ln/U, Cp = C₅H₄SiMe₃ (Cp’) or 1,3-(SiMe₃)₂C₅H₃(Cp”)) and [An^IICp”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 [An^IICp”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 5fⁿ6d¹and 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.

K. Randall McClain

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