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 (X^1-) both affected uranium redox reactivity, with more polarizing M¹⁺ and larger X^1- favoring uranium in low oxidation states. Changing M¹⁺ (Li, Na, K) shifted the U^IV + e^1- ⇌ U^III (U^IV/III) and U^III + 3e^1- → U⁰_metal potentials by +330 and +240 mV, respectively. Changing X^1- (Cl, Br, I) caused larger shifts of +440 mV for the U^IV/III redox potential and +1060 mV for the U⁰_metal deposition potential. Using Coulomb's Law, we correlated these potentials with electrostatic interactions between U^III and the molten salt. This model provided a facile way of predicting redox chemistry within molten salts.