THEORETICAL-STUDIES OF INORGANIC AND ORGANOMETALLIC REACTION-MECHANISMS .12. INTRAMOLECULAR CARBON-HYDROGEN BOND ACTIVATION IN (BUTENYL)MANGANESE TRICARBONYL

Theoretical treatment of the fluxional behavior exhibited by (butenyl) manganese tricarbonyl, (C4H7)Mn(CO)(3), is challenging because the str ucture contains a first-row transition metal and an agostic interactio n between the butenyl fragment and the manganese center. (C4H7)Mn(CO)( 3) displays two fluxional processes on the NMR time scale. The first a verages the hydrogens on the agostic carbon and has an activation free energy of Delta G(not equal) = 9.1 kcal/mol. We have identified the t ransition state in this process and calculated an activation free ener gy of Delta G(theor)(not equal) = 8.43 kcal/mol. In the transition sta te structure, the agostic bond is broken and the methyl group rotated. The second process averages the two halves of the syn-butadiene fragm ent and has an activation free energy of Delta G(theor)(not equal) = 1 7.1 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 Delta G(theor)(not equal) = 17.2 kcal/mol. Geometries for the ground state, intermediate, and transition states were optimized at the Molle r-Plesset second order perturbation theory and/or density functional t heory (DFT) levels. The density functional results were superior to th ose obtained by MP2. Final energetics were calculated by quadratic con figuration interaction on the DFT geometries in a basis set that conta ins polarization functions, with corrections for zero-point energy and temperature.