Slow uptake of molecular dihydrogen by the diiridium(I) prototype [Ir (-muz)(PPh3)(CO)](2) (1: pzH = pyrazole) is accompanied by formation of a 1,24hydrido-diiridium(II) adduct [IrH(mu-pz)(PPh3)(CO)](2) (2), for which an X-ray crystal structure determination reveals that (unlike in 1) the PPh3 ligands are axial, with the hydrides occupying trans coequatorial positions across the Ir-Ir bond (2.672 Angstrom). Reaction with CCl4 effects hydride replacement in 2, affording the monohydride Ir2H(Cl)(mu-pz)(2)(PPh3)(2)(CO)(2) (3) in which Ir-Ir = 2.683 Angstrom. At one metal center, H is equatorial and PPh3 is axial, while at the other, Cl is axial as is found in the symmetrically substituted product [Ir(mu-pz)(PPh3)(CO)Cl](2) (4) (Ir-Ir = 2.754 Angstrom) that is formed by action Of CCl4 on 1. Treatment of 1 with I-2 yields the diiodo analogue 5 of 4, which reacts with LiAlH4 to afford the isomorph Ir2H(I)(mu-pz)(2)(PPh3)(2)(CO)(2) (6) of 3 (Ir-Ir = 2.684 Angstrom). Protonation (using HBF4) of 1 results in formation of the binuclear cation Ir2H(mu-pz)(2)(PPh3)(2)(CO)(2)(+) (7: BF4- salt), which shows definitive evidence (from NMR) for a terminally bound hydride in solution (CH2Cl2 or THF), but 7 crystallizes as an axially symmetric unit in which Ir-Ir = 2.834 Angstrom. Reaction of 7 with water or wet methanol leads to isolation of the cationic diiridium(III) products [Ir2H2(mu-OX)(mu-pz)(2)(PPh3)(2)(CO)(2)]BF4 (8, X = H; 9, X = Me).