The electronic influence of ancillary ligands on the addition of H-2 to IrXH2(PR3)(2) (X = Cl, Br, I; R = H, Me) has been studied. The reaction enthalpy and the activation energy for the formation of a molecular dihydrogen complex have been computed. Results from ab initio MO perturbation theory (MP2) and density functional theory (B3LYP) are compared. The calculated geometries are in good agreement with the experimental results for IrClH2((PBu2Me)-Bu-t)(2) and IrIH2(H-2)((PPr3)-Pr-l)(2). The binding energy of H-2 is underestimated by B3LYP, while it is overestimated by MP2. The results are improved by modeling the phosphine with PMe3 instead of PH3. The reaction enthalpies and the activation energies are in excellent agreement with the experimental data and follow the experimental trend along the halide series. The transition state for the addition corresponds to the deformation of the unsaturated fragment IrXH2(PH3)(2) with almost no interaction with the incoming H-2 molecule. The trends for the activation and thermodynamic enthalpies among the halide series are rationalized in terms of pi donor ability of the halides and variable strength of the trans influence of the hydride ligand.