INFLUENCE OF ANCILLARY LIGANDS ON THE KINETICS AND THE THERMODYNAMICSOF H-2 ADDITION TO IRXH2(PR3)(2) (X = CL, BR, I AND R = H, ME) - COMPARISON BETWEEN DENSITY-FUNCTIONAL THEORY AND PERTURBATION-THEORY

The electronic influence of ancillary ligands on the addition of H-2 t o IrXH2(PR3)(2) (X = Cl, Br, I; R = H, Me) has been studied. The react ion enthalpy and the activation energy for the formation of a molecula r dihydrogen complex have been computed. Results from ab initio MO per turbation theory (MP2) and density functional theory (B3LYP) are compa red. The calculated geometries are in good agreement with the experime ntal 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 phosphi ne with PMe3 instead of PH3. The reaction enthalpies and the activatio n energies are in excellent agreement with the experimental data and f ollow the experimental trend along the halide series. The transition s tate for the addition corresponds to the deformation of the unsaturate d fragment IrXH2(PH3)(2) with almost no interaction with the incoming H-2 molecule. The trends for the activation and thermodynamic enthalpi es among the halide series are rationalized in terms of pi donor abili ty of the halides and variable strength of the trans influence of the hydride ligand.