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.