Stereoselective [4+2] Diels-Alder cycloadditions occur between unsaturated tertiary phosphine, phosphole, and arsine molybedenum carbonyl complexes, e.g., (Ph2E-trans-CH2CH=CHCH3)Mo(CO)(5) [E = P, As] and 3, 4-dimethyl-1-pbenylphosphole (DMPP). The coordinated DMPP undergoes a [1, 5] phenyl migration at about 145 degrees C Frier to or concomitant with the [4+2] intramolecular cycloaddition reaction with the alkene moiety of the dienophile to produce a new class of conformationally rigid bidentate ligands containing the 1-phosphanorbornene bicyclic ring system. The characteristic H-1, C-13{H-1}, and P-31{H-1} NMR spectroscopic features of these compounds are described, Crystal structures of most of the new compounds are reported.