Ab initio molecular orbital methods have been used to study transition state properties for the concerted addition reaction of H2 to Vaska-type complexes, trans-Ir(L)2(CO)X, 1 (L = PH3 and X = F, Cl, Br, I, CN, or H; L = NH3 and X = Cl). Stationary points on the reaction path retaining the trans-L2 arrangement were located at the Hartree-Fock level using relativistic effective core potentials and valence basis sets of double-ζ quality. The identities of the stationary points were confirmed by normal mode analysis. Activation energy barriers were calculated with electron correlation effects included via Moller-Plesset perturbation theory carried fully through fourth order, MP4(SDTQ). The more reactive complexes feature structurally earlier transition states and larger reaction exothermicities, in accord with the Hammond postulate. The experimentally observed increase in reactivity of Ir(PPh3)2(CO)X complexes toward H2 addition upon going from X = F to X = I is reproduced well by the calculations and is interpreted to be a consequence of diminished halide-to-Ir π-donation by the heavier halogens. Computed activation barriers (L = PH3) range from 6.1 kcal/mol (X = H) to 21.4 kcal/mol (X = F). Replacing PH3 by NH3 when X = Cl increases the barrier from 14.1 to 19.9 kcal/mol. Using conventional transition state theory, the kinetic isotope effects for H2/D2 addition are computed to lie between 1.1 and 1.7 with larger values corresponding to earlier transition states. Judging from the computational data presented here, tunneling appears to be unimportant for H2 addition to these iridium complexes.
|Number of pages||7|
|Journal||Journal of Physical Chemistry|
|Publication status||Published - 1993|
ASJC Scopus subject areas
- Physical and Theoretical Chemistry