Vibrational data (IR, Raman and inelastic neutron scattering) and a supporting normal coordinate analysis for the complex trans-W(CO)3(PCy3)2(η2-H2) (1) and its HD and D2 isotopomers are reported. The vibrational data and force constants support the well-established η2-bonding mode for the H2 ligand and provide unambiguous assignments for all metal-hydrogen stretching and bending frequencies. The force constant for the HH stretch, 1.3 mdyn/Å, is less than one-fourth the value in free H2 and is similar to that for the WH stretch, indicating that weakening of the H-H bond and formation of W-H bonds are well along the reaction coordinate to oxidative addition. The equilibrium isotope effect (EIE) for the reversible binding of dihydrogen (H2) and dideuterium (D2) to 1 and 1-d2 has been calculated from measured vibrational frequencies for 1 and 1-d2. The calculated EIE is 'inverse' (1-d2 binds D2 better than 1 binds H2 With K(H)/K(D) = 0.78 at 300 K. The EIE calculated from vibrational frequencies may be resolved into a large normal mass and moment of inertia factor (MMI = 5.77), an inverse vibrational excitation factor (EXC = 0.67), and an inverse zero-point energy factor (ZPE = 0.20), where EIE = MMI x EXC x ZPE. An analysis of the zero-point energy components of the EIE shows that the large decrease in the HH stretching frequency (force constant) predicts a large normal EIE but that zero-point energies from five new vibrational modes (which originate from translational and rotational degrees of freedom from hydrogen) offset the change in zero-point energy from the H2(D2) stretch. The calculated EIE is compared to experimental data obtained for the binding of H2 or D2 to Cr(CO)3(PCy3)2 over the temperature range 12-36 °C in THF solution. For the binding of H2 ΔH = -6.8 ± 0.5 kcal mol-1 and ΔS = -24.7 ± 2.0 cal mol-1 deg-1; for (D)2 ΔH = -8.6 ± 0.5 kcal/mol and ΔS = -30.0 ± 2.0 cal/(mol deg). The EIE at 22 °C has a value of K(H)/K(D) = 0.65 ± 0.15. Comparison of the equilibrium constants for displacement of N2 by H2 or D2 in the complex W(CO)3(PCy3)2(N2) in THF yielded a value of K(H)/K(D) = 0.70 ± 0.15 at 22 °C.