Uranium and thorium hydride complexes as multielectron reductants: A combined neutron diffraction and quantum chemical study

The unusual uranium reaction system in which uranium(4+) and uranium(3+) hydrides interconvert by formal bimetallic reductive elimination and oxidative addition reactions, [(C ₅Me ₅) ₂UH ₂] ₂ (1) ⇌ [(C ₅Me ₅) ₂UH] ₂ (2) + H ₂, was studied by employing multiconfigurational quantum chemical and density functional theory methods. 1 can act as a formal four-electron reductant, releasing H ₂ gas as the byproduct of four H ₂/H ^- redox couples. The calculated structures for both reactants and products are in good agreement with the X-ray diffraction data on 2 and 1 and the neutron diffraction data on 1 obtained under H ₂ pressure as part of this study. The interconversion of the uranium(4+) and uranium(3+) hydride species was calculated to be near thermoneutral (∼-2 kcal/mol). Comparison with the unknown thorium analogue, [(C ₅Me ₅) ₂ThH] ₂, shows that the thorium(4+) to thorium(3+) hydride interconversion reaction is endothermic by 26 kcal/mol.