ChemInform Abstract: Thermodynamics for the Hydrogenation of Dimanganese Decacarbonyl.

Abstract

The equilibrium constant for the hydrogenation of Mn{sub 2}(CO){sub 10} in supercritical carbon dioxide has been measured under carbon monoxide and hydrogen at total system pressures of 142-300 atm by in situ {sup 55}Mn and {sup 1}H NMR spectroscopy over the temperature range 165-220C. The hydrogenation of Mn{sub 2}(CO){sub 10} is efficiently promoted by the addition of Co{sub 2}(CO){sub 8} to the system. The rate of HMn(CO){sub 5} production at 100C is increased by 2 orders of magnitude with the addition of 1 equiv of Co{sub 2}(CO){sub 8}. The magnitude of this promotional effect has extended the range of temperatures down to 80C over which it is feasible to determine the equilibrium constant for Mn{sub 2}(CO){sub 10} hydrogenation. Van't Hoff plots for the hydrogenation of Mn{sub 2}(CO){sub 10} in the presence and absence of the Co{sub 2}(CO){sub 8} promoter were found to yield standard enthalpy and entropy changes that agree to within the statistical error limit of the equilibrium constant measurements. The resultant analysis for the combined data set yields {Delta}H{degree} = 8.7 {plus minus} 0.3 kcal/mol and {Delta}S{degree} = 8.5 {plus minus} 0.8 cal/(K{center dot}mol) for the hydrogenation of Mn{sub 2}(CO){sub 10}. The heterobimetallic dimer Mn(CO){sub 5}-Co(CO){sub 4} ismore » observed as an additional reaction product in both the {sup 55}Mn and the {sup 59}Co NMR spectra, within the mixed-metal system. The thermodynamics for the redistribution reaction yielding 2 equiv of MnCo(CO){sub 9} from the homonuclear dimers, Mn{sub 2}(CO){sub 10} and Co{sub 2}(CO){sub 8} was found to be nearly thermal neutral exhibiting {Delta}H{degree} = 0.8 {plus minus} 0.3 kcal/mol and {Delta}S{degree} = 0.7 {plus minus} 0.8 cal/(K{center dot}mol), respectively.« less