Experimental observations, together with a theoretical analysis, indicate that the energetics of the oxidative addition of Hz to the six-coordinate molybdenum 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 provides a striking contrast to that reported for oxidative addition of Wt to the Vaska system, trans-Ir(PPh3)(2)(CO)X. A theoretical analysis suggests that 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 other. Oxidative addition is thus favored sterically for the fluoride derivatives since the increased steric interactions upon forming the eight-coordinate complexes M(PMe3)(4)H2X2 would be minimized for the smallest halogen. The electronic component of the energetics is associated with the extent that pi-donation from X raises the energy of the doubly occupied 3e*, pi-antibonding, d(xz) and d(yz) pair of orbitals in trans-M(PMe3)(4)X-2. Consequently, with F as the strongest pi-donor, trans-M(PMe3)(4)X-2 is destabilized with respect to M(PMe3)(4)H2X2 by p(pi)-d(pi) interaction to the greatest extent for the fluoride complex, so that oxidative addition becomes most favored 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 energy (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 substantially greater than the W-H BDE, to the extent that the oxidative addition reaction is characterized by an inverse equilibrium deuterium isotope effect [K-H/K-D = 0.63(5) at 60 degrees C]. The inverse nature of the equilibrium isotope effect is associated with the large number (six) of isotope-sensitive vibrational modes in the product, compared to the single isotope-sensitive vibrational mode in reactant Hz. A mechanistic study reveals that the latter reaction proceeds via initial dissociation of PMe3, followed by oxidative addition to five-coordinate [W(PMe3)(3)I-2], rather than direct oxidative 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 that involves dissociation of PMe3 and elimination from the seven-coordinate species [W(PMe3)(3)H2I2].