Mechanistic and Theoretical Analysis of the Oxidative Addition of H2 to Six-Coordinate Molybdenum and Tungsten Complexes M(PMe3)4X2 (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 that the energetics of the oxidative addition of H2 to the six-coordinate molybdenum and tungsten complexes trans-M(PMe3)4X2 (M = Mo, W; X = F, Cl, Br, 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 provides a striking contrast to that reported for oxidative addition of H2 to the Vaska system, trans-Ir(PPh3)2(CO)X. A theoretical analysis suggests that the halide dependence for trans-M(PMe3)4X2 is a result of both steric and electronic factors, the components of which serve to reinforce each other. Oxidative addition is thus favored sterically for the fluoride derivatives since the increased steric interactions upon forming the eight-coordinate complexes M(PMe3)4H2X2 would be minimized for the smallest halogen. The electronic component of the energetics is associate...