IrHCl[sub 2]P[sub 2] (P = P[sup i]Pr[sub 3]) reacts rapidly with H[sub 2] at 25[degrees]C to set up an equilibrium where H[sub 2] binds trans to the original hydride ligand (trans-2). A second slower reaction forms IrH(H[sub 2])Cl[sub 2]P[sub 2] (cis-2), where the cis disposition of the chlorides, and also H cis to H[sub 2], was established by neutron diffraction. This molecule (unlike trans-2), shows rapid site exchange between coordinated H and H[sub 2]. cis-2 can be induced to lose HCL to form Ir(H)[sub 2]ClP[sub 2]. The structure of Ir(H)[sub 2]Cl(P[sup t]Bu[sub 2]Ph)[sub 2], an analog of 3, was shown by neutron diffraction to have a planar H[sub 2]IrCl in a Y shape, with Cl at the base of the Y and a H-Ir-H angle of only 73[degree]. ECP ab initio calculations of IrH[sub 2]Cl(PH[sub 3])[sub 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(H[sub 2])Cl[sub 2](PH[sub 3])[sub 2] is less stable than the cis-2 isomer by 10.3 kcal/mol. The Ir-H[sub 2] interaction is stronger in cis-2. The rotational barrier has been calculated in the two isomers as 2.3 (trans) andmore » 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. 74 refs., 12 figs., 5 tabs.« less