Quantifying Outer- and Inner-Coordination Sphere Effects Using Uranium Redox Chemistry in Molten Salt Solutions

Defining the relative influence of intramolecular and intermolecular forces is a fundamental problem in chemistry that is difficult to quantify. To address this challenge, we developed a method to evaluate the relative impact of direct chemical bonding in the inner-coordination sphere vs effects from cations in the outer-coordination sphere by comparative analysis of uranium redox reactivity in various molten salts. We observed that outer-coordination sphere cations (M¹⁺) and inner-coordination sphere anions (X¹⁻) both affected uranium redox reactivity, with more polarizing M¹⁺ and larger X¹⁻ favoring uranium in low oxidation states. Changing M¹⁺ (Li, Na, K) shifted the UIV + e¹⁻ ⇌ UIII (UIV/III) and UIII + 3e¹⁻ → U⁰metal potentials by +330 and +240 mV, respectively. Changing X¹⁻ (Cl, Br, I) caused larger shifts of +440 mV for the UIV/III redox potential and +1060 mV for the U⁰metal deposition potential. Using Coulomb's Law, we correlated these potentials with electrostatic interactions between UIII and the molten salt. This model provided a facile way of predicting redox chemistry within molten salts.