Sadhu Biswajit, Dolg Michael
Health Physics Division, Health Safety & Environment Group , Bhabha Atomic Research Center (BARC) , Mumbai 400 085 . India.
Institute for Theoretical Chemistry , University of Cologne , Greinstrasse 4 , 50939 Cologne , Germany.
Inorg Chem. 2019 Aug 5;58(15):9738-9748. doi: 10.1021/acs.inorgchem.9b00705. Epub 2019 Jul 25.
Soft donor ligands often provide higher selectivity for actinides(III) over chemically similar lanthanides(III), e.g., in the Am-Eu pair. Frequently, the origin of such selectivity is associated with an increased covalency in actinide-ligand bonding. However, the relationship between the degree of covalency and ion selectivity has yet to reach general consensus. Further, it is unclear whether the enhanced covalency leads to a thermodynamic stabilization of the complex or not. Using relativistic density functional theory, we have addressed these outstanding issues by analyzing the subtle change of metal-ligand interactions from a hard donor ligand to a mixed soft-hard one. The present comparative study on the structure of and binding in Am and Eu complexes with 3,4,3-LI(1,2-HOPO) (L) and its mixed-donor variant (LS) shows that the introduction of sulfur as a soft donor atom into the metal coordination sphere indeed infuses an Am selectivity into the otherwise nonselective ligand L but also leads to a significant reduction of the metal-binding Gibbs free energies. Natural population analysis, charge decomposition analysis, and its extended version point to the critical role of ligand-to-metal charge transfer in the overall thermodynamic stability of the complexes. A detailed energy decomposition analysis combining the extended transition state with the natural orbitals chemical valence method reveals an enhancement of the covalency upon switching to the soft-hard donor ligand because of the different nature of the metal-ligand interaction. The ligand L predominantly binds the metal via π donation, whereas the ligand LS prefers σ donation. Molecular orbital and quantum theory of atoms in molecules analyses as well as a comparison to a simple model system show that the covalency occurs as a result of orbital mixing and is near-degeneracy-driven in nature. This enhanced covalency, however, fails to thermodynamically compensate for the loss of strong electrostatic interaction and thus does not lead to an additional stabilization of the metal-LS complexes.
软供体配体通常对锕系元素（III）的选择性高于化学性质相似的镧系元素（III），例如在镅 - 铕对中。通常，这种选择性的来源与锕系元素 - 配体键合中增加的共价性有关。然而，共价程度与离子选择性之间的关系尚未达成普遍共识。此外，尚不清楚增强的共价性是否会导致配合物的热力学稳定性增强。使用相对论密度泛函理论，我们通过分析从硬供体配体到软硬混合配体时金属 - 配体相互作用的细微变化，解决了这些突出问题。目前对镅和铕与3,4,3 - LI(1,2 - HOPO)（L）及其混合供体变体（LS）形成的配合物的结构和键合的比较研究表明，将硫作为软供体原子引入金属配位球中确实赋予了原本无选择性的配体L对镅的选择性，但同时也导致金属结合吉布斯自由能显著降低。自然布居分析、电荷分解分析及其扩展版本表明，配体到金属的电荷转移在配合物的整体热力学稳定性中起关键作用。结合扩展过渡态与自然轨道化学价方法的详细能量分解分析表明，由于金属 - 配体相互作用性质不同，切换到软硬供体配体时共价性增强。配体L主要通过π给予与金属结合，而配体LS更倾向于σ给予。分子轨道和分子中原子的量子理论分析以及与简单模型系统的比较表明，共价性是轨道混合的结果，本质上是近简并驱动的。然而，这种增强的共价性未能在热力学上补偿强静电相互作用的损失，因此不会导致金属 - LS配合物的额外稳定。
