Variable-temperature C-13 NMR does not show any evidence for intramolecular ethylene rotation in 1-C-13. The rates of alkene dissociation for the propene (8) and trans-2-butene (7) adducts of Os-2(CO)(8) have been measured in hydrocarbon solution and compared with the rates of alkene dissociation from the corresponding Os(CO)(4)(alkene) adducts 6 and 9. The kinetic labilities of propene and trans-2-butene are reversed in the Os-2(CO)(8)(alkene) and Os(CO)(4)(alkene) systems; propene is replaced 2.5 times faster than trans-2-butene in the Os-2(CO)(8)(alkene) system, while trans-2-butene is replaced 55.9 times faster than propene in the Os(CO)(4)(alkene) system. We have used molecular mechanics to explore the reasons for this unusual reactivity pattern and have found that these results may be easily reconciled with a ring-opening mechanism for alkene replacement in the Os-2(CO)(8)(alkene) system. We have confirmed that alkene exchange with Os(CO)4(alkene) is dissociative, in agreement with precedent. The secondary deuterium kinetic isotope effect (KIE) has been measured for the replacement of C2H4 and C2D4 in Os-2(CO)(8)(mu-eta(1),eta(1)-C2H4) (1) and Os-2(CO)(8)(mu-eta(1),eta(1)-C2D4) (1-d(4)); it is 1.30(1) at 39 degrees C. The measured KIE is consistent with a ring-opening associative mechanism for alkene exchange (mechanism II in the previous paper).