Date of Award
Master of Science (MS)
Department of Chemistry
David L. CedeÃ±o
Metal-olefin equilibrium geometries, bond formation energies (Î?E), enthalpies (Î?H), and free energies (Î?G) for a select series of transition (M = Ni, Fe, Cr, Mo and W) metal-olefin carbonyl complexes [M(CO)x(Æ?2-C2H3-C6H4-Y)] have been calculated and compared using density functional theory (DFT), with the BP86 functional under standard state conditions (1 atm, 298.15 K) for the general gas phase formation reaction:
M(CO)x + (C2H3-C6H4-Y) â?? [M(CO)x(Æ?2-C2H3-C6H4-Y)] (1)
Y = NO2, CN, COOH, H, OH, NH2, N(CH3)2
In regards to the electronic modification of the substituent (Y) on styrene at the para position, this study quantitatively investigated the effect of electron-withdrawing and electron-donating influence on transition metal-olefin coordination. All complex geometries were optimized to minimum energy conformations. Geometric results show evidence of sp2 to sp3 rehybridization of the olefin carbon atoms. Metal-olefin bond energies were evaluated using a bond energy decomposition analysis (BEDA) scheme.
The key attractive and repulsive interactions contributing to the bond formation energies were obtained from the BEDA. The trends were compared with those expected from the traditional Dewar-Chatt-Duncanson (DCD) frontier orbital bonding model. The DCD model was not always predictive of the bond energy strengths, since it does not consider thermodynamic costs from geometrical changes. An energy decomposition analysis of the bonding interactions demonstrate that, contrary to the DCD bonding model, as electron-withdrawing nature of the para substituent increase, strength of the metal-olefin interaction diminishes.
Density functional theory has also been applied to describe electronic substituent effects, especially in the pursuit of linear relationships similar to those observed from the Hammett Correlations based on Linear Free Energy Relationships (LFERs). Plots of Log (k/kH) vs. various Hammett parameters based on ionization of benzoic acids (Ï?p) indicate that the rate of metal-olefin bond formation occurs faster from complexes with more electron-donating capacity for the [M(CO)5-L-Y] complex series. Whereas, rates of bond formation for the [Fe(CO)4-L-Y] and[Ni(CO)3-L-Y] complex series were much less sensitive to substituent effects based on acquired reaction constants Ï?.
Berninger, Michael John, "Transition Metal-Olefin Bonding Interactions: a Density Functional Theory Study [ M ( Co ) X ( Æ? 2-C 2 H 3-C 6 H 4-Y) ]" (2015). Theses and Dissertations. 455.