IrHCl2P2 (P = PiPr3) reacts rapidly with H2 at 25 °C to set up an equilibrium where H2 binds trans to the original hydride ligand (trans-2). A second slower reaction forms IrH(H2)Cl2P2 (cis-2), where the cis disposition of the chlorides, and also H cis to H2, was established by neutron diffraction. This molecule (unlike trans-2), shows rapid site exchange between coordinated H and H2. cis-2 can be induced to lose HCl to form Ir(H)2ClP2 (3). The structure of Ir(H)2Cl(PtBu2Ph)2, an analog of 3, was shown by neutron diffraction to have a planar H2IrCl in a Y shape, with Cl at the base of the Y and a H-Ir-H angle of only 73°. ECP ab initio calculations of IrH2Cl(PH3)2 show that the Y shape with a H-Ir-H angle close to the experimental value has the minimum energy. They also show that the trans-2 isomer of IrH(H2)Cl2(PH3)2 is less stable than the cis-2 isomer by 10.3 kcal/mol. The Ir-H2 interaction is stronger in cis-2. The rotational barrier has been calculated in the two isomers as 2.3 (trans) and 6.5 (cis) kcal/mol. In agreement with the experimental structure, the H-H bond is found to eclipse preferentially the Ir-H bond in cis-2. The calculations also show that the Ir-H2 bond dissociation energy is greater in cis-2. It thus appears that the binding ability of a metal fragment not only depends on its ligands but is also linked in a subtle way to its stereochemistry. The J(HD) value for coordinated H2 in cis-2 is 12 ± 3 Hz. The implication of this small value and of a T1min(200 MHz) of 38 ms is an H/H distance of 1.07–1.35 Å, which compares to the neutron diffraction distance of 1.11(3) Å. The Ir-H distances of cis-2 are unprecedented in that the hydride-Ir distance (1.584(13) Å) is not shorter than the distances to the H2 hydrogens (1.537(19) and 1.550(17) Å). One of the H2 hydrogens interacts with chloride of an adjacent molecule to give an infinite hydrogen-bonded polymer. An inelastic neutron scattering spectroscopic study on solid IrHCl2(H2)(PiPr3)2 sets a lower limit on the rotational barrier of the Ir(H2) unit of 2.0 kcal/mol. Ab initio calculations on IrHCl2(H2)(PH3)2 yield a H-H distance in these two isomers of 0.81 and 1.4 Å, respectively, showing that the moiety IrHCl2(PH3)2 with chlorides mutually cis is a much stronger reducing agent than that with chlorides trans (and thus H trans to H2). Crystallographic data: For cis-2 (at 15K), a = 13.008(4) Å,b= 11.296 (4) Å, c = 16.095(4) Å in space group Pna21 (Z = 4). For Ir(H)2Cl(PtBu2Ph)2 (at 15K), a = 8.236(2) Å, b = 17.024(6) Å, c = 20.528(10) Å, β = 96.27(4)° in space group P21/c (Z = 4).