The two diastereomers of Mo(NAr)(CHCMe2Ph)(2,5-dimethylpyrrolide)(1), (SMR1)-2 and (RMR1)-2, respectively, where 1 is an enantiomericatly pure (R) phenoxide and Ar = 2,6-diisopropylphenyl, form adducts with PMe3. One of these ((RMR1)-2(PMe3)) has been isolated. An X-ray structure reveals that PMe3 has added trans to the pyrrolide; it is a model for where an olefin would attack the metal. Trimethylphosphine will catalyze slow interconversion of (SMR1)-2 and (RMR1)-2 via formation of weak PMe3 adducts. Reactions between (SMR1)-2 or (RMR1)-2 and ethylene yield Mo(NAr)(CH2)(Me(2)Pyr)(1) species in which the configuration at Mo is inverted by ethylene at a rate of the order of the NMR time scale at 22 degrees C via formation of metallacyclobutane intermediates with imido and aryloxide ligands in axial positions. A reactant olefin is proposed to approach Mo and the product otefin to leave Mo trans to the pyrrolide.