Two-electron mixed-valence bimetallic cores of iridium are stabilized in a diphosphazane, MeN-[P(OCH2CF3)(2)](2) (bis(bis(trifluoroethoxy)phosphino)methylamine, tfepma) coordination sphere. Treatment of [Ir(cod)Cl](2) with tfepma affords the Ir-2(0,II) complex, Ir-2(tfepma)(3)Cl-2 (1), in which the Ir-0 and Ir-II centers assume trigonal bipyramidal and square pyramidal coordination geometries, respectively. The coordinatively unsaturated two-electron mixed-valence core of 1 supports an extensive acid-base and oxidation-reduction chemistry. As established by single crystal X-ray analysis, two-electron donor ligands are readily received at the Ir-II center of 1 to complete the octahedral coordination environment that is preferred by a d(7) metal-metal bonded center. Alternatively, redox-active substrates rapidly add across the single metal-metal bond of 1 to form Ir-2(I,III) mixed-valence complexes; the chlorine and hydrochloric acid adducts, Ir-2(tfepma)(3)Cl-4 (5) and Ir-2(tfepma)(3)-HCl3 (7b), respectively, have been characterized by NMR spectroscopy and X-ray crystallography. Likewise, H-2 reacts with 1 to afford an Ir-2(I,III) dihydride complex, Ir-2(tfepma)(3)H2Cl2 (8). Single-crystal X-ray and NMR analyses of 8 reveal that a single hydride ligand is coordinated at each iridium center of the bimetallic core. Hydrogen is readily removed from the complex in solution and the solid state, providing the first example of the reversible addition of dihydrogen across a single metal-metal bond.