Probing <i>f</i> -Block Covalency at the Limits of Hard-Metal/Soft-Ligand Interactions through Chalcogenoether Complexes

We report a systematic probe of covalency at the boundaries of <i>f</i>-block hard-metal/soft-ligand interactions by employing monoanionic ligands containing formally neutral chalcogen donors to facilitate isostructural comparisons of early to middle trivalent actinides (An = U, Np, Pu, Am) and their near isoradial lanthanide counterparts (Ln = La, Ce, Pr, Nd). The single-crystal structural data on [(AE₂^Ph2)₂M^IIII] (<b>1-M</b>, E = S, M = La, Ce, Pr, Nd, U, Np, Pu, Am; <b>2-M</b>, E = Se, M = La, Ce, Pr, Nd, Np, Pu, Am; AE₂^Ph2 = 4,5-bis(phenylchalcogenido)-2,7,9,9-tetramethylacridanide) provides metrical definition of the first transuranium actinide selenoether bonds to Np, Pu, and Am as well as the first Pr and Nd selenoether complexes. Additionally, rare examples of transuranium thioether and lanthanum/cerium-selenoether complexes are reported. The shorter metal-chalcogen bonds in the actinide complexes compared to lanthanide congeners suggest stronger covalent interactions, though it becomes less pronounced as the <i>f</i>-block series is traversed to the Am vs Nd comparison. Computational analyses support these findings, showing increased metal-ligand participation in the U, Np, and Pu complexes and further participation of valence shell orbitals when moving from S to Se as the chalcogenoether donor atom.