[en] The synthesis and characterization of derivatives of Mo(CO)(R₂PC₂H₄PR₂)₂ (R = Et, i-Bu, Ph, Et-Ph) and their reactions with H₂, N₂, and SO₂ are reported. For R = Et and i-Bu, the H₂ oxidatively adds to give dihydrides, but for R = Ph, a η²-H₂ complex is formed. Single-crystal neutron diffraction of Mo(CO)(H₂)(Ph₂PC₂H₄PPh₂)₂ (as a 4.5-benzene solvate with all Ph groups deuterated) at 12 K showed the H-H bond to be oriented trans to the CO and parallel to a P-Mo-P axis, with a length close to that of free H₂ (0.74 Angstrom). However, the thermal ellipsoids were very large, and inelastic neutron scattering showed that the barrier to rotation of the H₂ is the lowest yet measured, ca. 0.7 kcal/mol. These observations indicate that librational motion of the H₂ is artificially foreshortening the H-H bond length. Application of a correction procedure gave a distance of 0.80-0.85 Angstrom as being more likely. Extended Huckel calculations successfully modeled the H₂ coordination and also showed a low rotational barrier (1.4 kcal/mol). Theoretical considerations suggest that the degree of distortion of the MP₄ skeleton is largely responsible for the ability of the complex to bind molecular hydrogen and controls the amount of back-bonding from the metal d-orbital to H₂ σ^*. The lack of an elongated H-H bond length or equilibrium with a dihydride tautomer, despite the apparent nearness of the H₂ to cleavage, leads to the conclusion that the reaction coordinate for oxidative addition of H₂ is rather flat until relatively precipitous cleavage of the H₂. Mo(CO)(H₂)[(C₆D₅)₂PC₂H₄P(C₆D₅)₂]₂·4.5C₆D₆ crystallizes in the space group P bar 1. 45 refs., 9 figs., 4 tabs