Bonding Trends in Pyridine-2-thiolato Complexes of Tetravalent Actinides

Complexes of tetravalent actinides (An: Th, U, Np, and Pu) with the bidentate (<i>N,S</i>)-donor ligand pyridine-2-thiolate (2-PyS, PyS^-) were synthesized in 1:4 or 1:5 ratios. This includes the first structurally characterized Np complex with (<i>N,S</i>)-donor ligands, filling a notable gap in the An coordination chemistry. An improved synthetic approach with PyS-SiMe₃ enabled efficient formation of the 1:4 complexes in THF as a coordinating solvent. The compounds were comprehensively characterized in solution and in the solid phase, supported by quantum chemical calculations. Experimental and theoretical results show matching trends in the binding behavior of An^IV. The covalent bond contributions in An-N and An-S bonding increase along the series of An from Th to Pu. The bonds to the soft sulfur donors consistently have the highest covalent contributions with a remarkably high percentage for Pu-S (IQA analysis: >34%) over the harder N donors or An-O bonds of coordinating THF. High-resolution X-ray absorption and infrared spectroscopy indicate similar electronic and structural properties across An^IV complexes, while SQUID magnetometry uncovered significant differences in magnetic behavior at low temperatures depending on the complex compositions. This work advances the understanding of An-ligand bonding, emphasizing covalency and electronic structures, and expands fundamental insights into An chemistry.