The reaction of trans-[M(C₂H₄)₂(PMe₃)₄] with the tripodal phosphine 1,1,1-tris[(dimethylphosphino)methyl]ethane (CP₃) affords complexes trans-[M(C₂H₄)₂(η²-CP₃)(PMe₃)₂] (M = Mo 1a or W 1b). A detailed NMR study, including hetero- and homo-nuclear two-dimensional correlations, has been carried out for 1b which allows a complete assignment of signals and a structural spectroscopic determination. In solution, no ethylene rotation is observed at room temperature and the preferential conformer of the six-membered ring W[η²-CH₃C(CH₂PMe₂)₃] fragment is, on the basis of steric arguments, the skew-boat conformation. Interaction of compounds 1 with CO affords trans-[M(C₂H₄)₂(η²-CP₃)(CO)(PMe₃)] (M = Mo 2a or W 2b), while reaction of 1a with CNBu^t furnishes the compound trans-[Mo(C₂H₄)₂(η²-CP₃)(CNBu^t)(PMe₃)] 3. The bidentate co-ordination of CP₃ induces chirality at the metal center and compounds 2 and 3 are obtained as a ca. 1∶1 mixture of diastereoisomers, identified by the presence of two different patterns of resonances in their ³¹P-{¹H} NMR spectra at low temperature. At the fast regimen exchange, the isomers are interconverted through bis(ethylene) rotation. A variable-temperature ³¹P-{¹H} NMR study carried out for 2a gives an approximate value of ΔG^‡ for this process of 55 kJ mol^–1 at 298 K.