Theoretical studies of inorganic and organometallic reaction mechanisms .12. Intramolecular carbon-hydrogen bond activation in (butenyl)manganese tricarbonyl
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Theoretical treatment of the fluxional behavior exhibited by (butenyl)manganese tricarbonyl, (C4H7)Mn(CO)3, is challenging because the structure contains a first-row transition metal and an agostic interaction between the butenyl fragment and the manganese center. (C4H7)Mn(CO)3 displays two fluxional processes on the NMR time scale. The first averages the hydrogens on the agostic carbon and has an activation free energy of G = 9.1 kcal/ mol. We have identified the transition state in this process and calculated an activation free energy of Gtheor = 8.43 kcal/mol. In the transition state structure, the agostic bond is broken and the methyl group rotated. The second process averages the two halves of the syn-butadiene fragment and has an activation free energy of G = 17.1 kcal/mol. We have identified the transition state and intermediate structures for this process in which the agostic hydrogen oxidatively adds to the metal center. The calculated free energy of activation is Gtheor = 17.2 kcal/mol. Geometries for the ground state, intermediate, and transition states were optimized at the Mller-Plesset second order perturbation theory and/or density functional theory (DFT) levels. The density functional results were superior to those obtained by MP2. Final energetics were calculated by quadratic configuration interaction on the DFT geometries in a basis set that contains polarization functions, with corrections for zeropoint energy and temperature. 1997 American Chemical Society.
