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. Electronic considerations, primarily back-bonding to H2 σ*, are shown to be the primary cause of H-H bond cleavage. 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 Å). 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 Å 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*. Mo(CO)(H2)(Ph2PC2H4PPh2)2 is now the fourth dihydrogen complex to show an H-H neutron distance of ca. 0.82 Å despite widely varying ligand sets, central metals, and charges. 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. The cell dimensions (X-ray, 233 K) are as follows: a = 13.038 (3) Å,b= 14.125 (3) Å, c = 19.995 (4) Å, α = 90.42 (3)°, β = 94.48 (3)°, γ = 113.83 (3)°, Z = 2; R = 4.2%.