The X-ray crystal structures of four closely related Mo(CO) 5(R2PXR′) (R2 = OCH2CMe 2CH2O, XR′ = S-2-Pr, NHC6H 4-4-Me; R2 = Ph2, XR′ = NHC 6H4-2-Me, OC6H4-4-SMe) complexes have been determined. The R2PXR′ ligands are oriented so that the P-X bond and one of the Mo-C bonds are nearly eclipsed. This results in the distortion of the octahedral coordination geometry via tilting of the Mo(CO)5 group away from the XR′ group. As observed in related complexes, the Mo-C bond of the carbonyl trans to the phosphorus-donor group is shorter than are the Mo-C bonds of the carbonyls trans to carbonyls. In contrast, no significant differences were observed between the Mo-C bonds of carbonyls trans to phosphites and the Mo-C bonds of carbonyls trans to phosphinites. The conformations of the 1,3,2-dioxaphosphorinanes were distorted chairs with the P end of the chair significantly flattened relative to the seat of the chair. This conformation is similar to that which has been observed for 1,3,2-dioxaphosphorinanes in other transition metal complexes.