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Reaction Mechanisms of Metal-Metal Bonded Carbonyls. X. Thermal Decomposition of Decacarbonyldimanganese and Decacarbonylmanganeserhenium in Decalin* J. Paul Fawcett, Anthony Poe,* and Kumud R. Sharma Contributionfrom Erindale College and the Department of Chemistry, University of Toronto, Toronto, Canada. Received July 18, 1974
Abstract. The thermal decomposition of decacarbonyldimanganese in decalin, in the absence of oxygen and over a range of oxygen concentrations, has been studied in some detail. In the absence of oxygen the reaction is half-order in [Mn2(CO)lo] over the range (2-350) X loT5M but decomposition under 02-Nz mixtures shows a transition from first- to half-order as M . The results are fully consistent with initial slow homolytic fission to form [Mn2(CO)lo] is increased from 1 to 500 X Mn(C0)s radicals that can rapidly either recombine or react further to form decomposition products. Similar behavior is found with decacarbonylmanganeserhenium, and the activation parameters for the rate-limiting steps therefore provide a kinetic measure of the strength of the metal-metal bonds.
The mechanisms of reactions of dimanganese decacarbonyl have been a matter of some uncertainty. Wawersik and Basolo studied several substitution reactions in xylene of the decacarbonyl and some of its simple derivatives and concluded that their results were fully consistent with a simple dissociative m e c h a n i ~ m Po& . ~ et al. studied substitution reactions of the decacarbonyl in decalin, and also the thermal decomposition under atmospheres of nitrogen, air, and pure ~ x y g e n Limiting .~ rate constants for substitution by triphenylphosphine were appreciably higher than those obtained under oxygen unless the substitutions were performed under an atmosphere of carbon monoxide when the limiting rate constants for the two reactions appeared to be identical at all temperatures. It was concluded that the substitutions do proceed in part by a CO-dissociative path (completely inhibited by 1 atm of carbon monoxide) but that an additional path is available for substitution and for thermal decomposition in the presence or absence of oxygen. It was proposed that this path involves significant Mn-Mn bond breaking in the transition state. Similar results were also obtained for reactions of dirhenium decacarbonyl but the importance of the CO-inhibited path for substitution was much less in that case.5 Since the thermal decomposition reactions appeared to provide evidence of a rather different kind from the substitutions, we have extended their study considerably, and have shown that a simple CO-dissociative mechanism cannot possibly explain the results. The data are, however, in excellent agreement with a mechanism involving initial, slow homolytic fission of the Mn-Mn bond to form M n ( C 0 ) s radicals, which can then either recombine or react further in some way. Similar results are also reported for reactions of decacarbonylmanganeserhenium.
Experimental and Results Baker Analyzed decalin was dried over molecular sieves and used without further treatment. Decacarbonyldimanganese (Strem Chemicals, Inc.) was recrystallized from hexane. Decacarbonylmanganeserhenium was prepared by the reaction6 of Re(C0)5CI with NaMn(C0)s in freshly distilled anhydrous tetrahydrofuran. Kinetic runs were performed by standard techniques,' degassing by repeated freeze-pump-thaw cycles, and the use of high purity gases (Argon, 99.998%. from Union Carbide of Canada, Ltd., and carbon monoxide,
