Computational study of the transition state for H₂ addition to Vaska-type complexes (trans-Ir(L)₂(CO)X): Substituent effects on the energy barrier and the origin of the small H₂/D₂ kinetic isotope effect

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.