Structural and Spectroscopic Comparison of Soft-Se vs. Hard-O Donor Bonding in Trivalent Americium/Neodymium Molecules.

Covalency is often considered to be an influential factor in driving An vs. Ln selectivity invoked by soft donor ligands. This is intensely debated, particularly the extent to which An /Ln covalency differences prevail and manifest as the f-block is traversed, and the effects of periodic breaks beyond Pu. Herein, two Am complexes, [Am{N(E=PPh ) } ] (1-Am, E=Se; 2-Am, E=O) are compared to isoradial [Nd{N(E=PPh ) } ] (1-Nd, 2-Nd) complexes. Covalent contributions are assessed and compared to U/La and Pu/Ce analogues. Through ab initio calculations grounded in UV-vis-NIR spectroscopy and single-crystal X-ray structures, we observe differences in f orbital involvement between Am-Se and Nd-Se bonds, which are not present in O-donor congeners.