COMPUTATIONAL STUDY OF THE TRANSITION-STATE FOR H2 ADDITION TO VASKA-TYPE COMPLEXES (TRANS-IR(L)2(CO)X) - SUBSTITUENT EFFECTS ON THE ENERGYBARRIER 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 H-2 to Vaska- type complexes, trans-lr(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 re taining the trans-L2 arrangement were located at the Hartree-Fock leve l using relativistic effective core potentials and valence basis sets of double-zeta quality. The identities of the stationary points were c onfirmed by normal mode analysis. Activation energy barriers were calc ulated with electron correlation effects included via Moller-Plesset p erturbation 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 postu late. The experimentally observed increase in reactivity of Ir(PPh3)2( CO)X Complexes toward H-2 addition upon going from X = F to X = I is r eproduced well by the calculations and is interpreted to be a conseque nce of diminished halide-to-Ir pi-donation by the heavier halogens. Co mputed activation barriers (L = PH3) range from 6.1 kcal/mol (X = H) t o 21.4 kcal/mol (X = F). Replacing PH3 by NH3 when X = Cl increases th e barrier from 14.1 to 19.9 kcal/mol. Using conventional transition st ate theory, the kinetic isotope effects for H-2/D2 addition are comput ed to lie between l.l and l.7 with larger values corresponding to earl ier transition states. Judging from the computational data presented h ere, tunneling appears to be unimportant for H-2 addition to these iri dium complexes.