Two-electron mixed-valence complexes of the general formula (tfepma)(3)lr(2)(0)'(RBr)-R-ll [tfepma = bis(bis(trifluoroethoxy)-phosphino)methylamine, MeN[P(OCH2CF3)(2)](2), and R = CH3 (2), CH2C(CH3)(3) (3)] have been synthesized and structurally characterized and their reactivity with H-2 investigated. Hydrogenation of 2 and 3 proceeds in a cascade reaction to produce alkane upon initial H-2 addition, followed by the formation of the lr(2)(I,III) binuclear trihydridebromide complex (tfepma)(3)lr(2)(I,III)H(3)Br (4) upon the incorporation of a second molecule of H-2. Hydrogenation of two-electron mixed-valence di-iridium alkyl complexes is examined with nonlocal density-functional calculations. H-2 attacks the Ir-II metal center prior to alkyl protonation to produce an eta(2) -H-2 complex. Transition states link all intermediates to a complex that has the same regiochemistry as the crystallographically determined final product. Calculated atomic charges suggest that the second H-2 molecule is homolytically cleaved within the di-iridium coordination sphere and that a hydrogen atom migrates across the intact Ir-Ir metal bond. These results are consistent with the emerging trend that two-electron mixed-valence cores manage the two-electron chemistry of substrates with facility when hydrogen is the atom that migrates between metal centers.