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Neutron-inelastic-scattering measurements have been made on a sample of highly purified solid C6o. New features in the spectrum of intramolecular modes not obtainable by optical spectroscopy have been identified. Available theoretical calculations show trends in accord with the experimental results but disagree in important details, and improved calculations are desirable.

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Variable-temperature 13C NMR does not show any evidence for intramolecular ethylene rotation in 1-13C. The rates of alkene dissociation for the propene (8) and trans-2-butene (7) adducts of Os2(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 Os2(CO)8(alkene) and Os(CO)4(alkene) systems; propene is replaced 2.5 times faster than trans-2-butene in the Os2(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 Os2(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 Os2(CO)8(μ-η1,η1-C2H4) (1) and Os2(CO)8(μ-η1,η1-C2D4) (1-d 4); it is 1.30(1) at 39 °C. The measured KIE is consistent with a ring-opening associative mechanism for alkene exchange (mechanism II in the previous paper).

Diosmacyclobutanes Os2(CO)8(olefin) undergo facile exchange reactions with olefins and acetylenes. An associative mechanism is excluded by the observation of saturation kinetics as the concentration of the entering olefin (butyl acrylate) is increased. Multivariate analysis of the rate as a function of [C2H4] (determined from solubility measurements in decane/butyl acrylate mixtures) and [butyl acrylate] suggests a stepwise mechanism in which the departing olefin migrates to a terminal position and then undergoes associative exchange with the entering olefin. The straightforward dissociative exchange of the olefin ligands in Os(CO)4(olefin) shows saturation at much lower concentrations of entering olefin.

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