Mechanistic and theoretical analysis of the oxidative addition of H-2 to six-coordinate molybdenum and tungsten complexes M(PMe3)(4)X-2 (M = Mo, W; X= F, Cl, Br, I): An inverse equilibrium isotope effect and an unprecedented halide dependence

Experimental observations, together with a theoretical analysis, indicate t hat the energetics of the oxidative addition of Hz to the six-coordinate mo lybdenum and tungsten complexes trans-M(PMe3)(4)X-2 (M Mo, W; X = F, Cl, Er , I) depend very strongly on the nature of both the metal and the halogen. Specifically, the exothermicity of the reaction increases in the sequences Mo < W and I < Br < Cl < F. Of most interest, this halogen dependence provi des a striking contrast to that reported for oxidative addition of Wt to th e Vaska system, trans-Ir(PPh3)(2)(CO)X. A theoretical analysis suggests tha t the halide dependence for trans-M(PMe3)(4)X-2 is a result of both steric and electronic factors, the components of which serve to reinforce each oth er. Oxidative addition is thus favored sterically for the fluoride derivati ves since the increased steric interactions upon forming the eight-coordina te complexes M(PMe3)(4)H2X2 would be minimized for the smallest halogen. Th e electronic component of the energetics is associated with the extent that pi-donation from X raises the energy of the doubly occupied 3e*, pi-antibo nding, d(xz) and d(yz) pair of orbitals in trans-M(PMe3)(4)X-2. Consequentl y, with F as the strongest pi-donor, trans-M(PMe3)(4)X-2 is destabilized wi th respect to M(PMe3)(4)H2X2 by p(pi)-d(pi) interaction to the greatest ext ent for the fluoride complex, so that oxidative addition becomes most favor ed for this derivative. Equilibrium studies of the oxidative addition of Hz to trans-W(PMe3)(4)I-2 have allowed the average W-H bond dissociation ener gy (BDE) in W(PMe3)(4)H2I2 to be determined [D(W-H) 62.0(6) kcal mol(-1)]. The corresponding average W-D BDE [D(W-D) = 63.8(7) kcal mol(-1)] is substa ntially greater than the W-H BDE, to the extent that the oxidative addition reaction is characterized by an inverse equilibrium deuterium isotope effe ct [K-H/K-D = 0.63(5) at 60 degrees C]. The inverse nature of the equilibri um isotope effect is associated with the large number (six) of isotope-sens itive vibrational modes in the product, compared to the single isotope-sens itive vibrational mode in reactant Hz. A mechanistic study reveals that the latter reaction proceeds via initial dissociation of PMe3, followed by oxi dative addition to five-coordinate [W(PMe3)(3)I-2], rather than direct oxid ative addition Co trans-W(PMe3)(4)I-2. Conversely, reductive elimination of H-2 does not occur directly from W(PMe3)4H2I2 but rather by a sequence tha t involves dissociation of PMe3 and elimination from the seven-coordinate s pecies [W(PMe3)(3)H2I2].