Metal−Olefin Bond Energies in M(CO)₅(C₂H_4-nCl_n) M = Cr, Mo, W; n = 0−4: Electron-Withdrawing Olefins Do Not Increase the Bond Strength

Abstract

Metal-olefin bond dissociation enthalpies have been calculated for the series of complexes M(CO)5(C2H(4-n)Cln), M = Cr, Mo, W; n = 0-4 using density functional theory. Experimental values of the bond enthalpies have been measured for M(CO)5(C2H(4-n)Cln) M = Cr, Mo, W; n = 2 (vinyl chloride), 3, and 4 using laser photoacoustic calorimetry in n-hexane solution. Experimental and calculated values indicate that the trend in metal-olefin bond energies is opposite to the electron-withdrawing ability of the olefin, which is counter to expectations based on the Dewar-Chatt-Duncanson model for metal-olefin bonding. An in-depth analysis of the metal-olefin interaction using a bond energy decomposition scheme implies that the observed and calculated decreasing trend is influenced by the increase in steric interactions and olefin reorganizational energy which is concomitant to the increase of the number of electron-withdrawing halogen atoms.

Transition Metal-Olefin Bonding Interactions: A Density Functional Theory Study [M(CO) x (Æ? 2 - C 2 H 3 - C 6 H 4 - Y)]

“…A number of important chemical processes such as olefin (alkene) hydrogenation, metathesis, polymerization, and oxidation among others are driven by the presence of a transition metal catalyst and involve the formation and or cleavage of a metal-olefin bond. [1][2][3][4][5][6][7][8][9][10][11] Thus, because the occurrence of olefin and olefin-related products in industry has become more prevalent, a more complete understanding of the factors that influence the strength of this bonding interaction must be developed in order to establish a more rational design of suitable catalysts for such processes.…”

“…31 DFT is regarded as a powerful tool for providing quantitative insights into metalolefin interactions that are difficult to study using experiments, as it allows for the fairly accurate calculation of bond energies. 1 As such, these calculations can be used to explain trends in bond dissociation enthalpy, to test available models for bonding, and to attempt to formulate more quantitative models of bonding. 24 A Bond Energy Decomposition Analysis (BEDA) scheme included in the Amsterdam Density Functional (ADF) chemistry software was used to quantify electronic, steric, and reorganizational interactions.…”

“…The interaction energy (ΔE int ) can then be further delineated as the sum of attractive ( DFT calculations and experimental data. 1,22,38 In this thesis, metal-olefin equilibrium geometries, bond formation energies (ΔE), enthalpies (ΔH), and free energies (∆G) for a select series of transition (M = Ni, Fe, Cr,…”

“…Since 1953, the Dewar-Chatt-Duncanson model has been utilized to rationalize and explain the nature of chemical bonding between an olefin ligand and a metal forming a π complex, as well as molecular geometry. 1 In 1953, an X-ray crystallographic analysis of Zeise's salt was collected and the structure of Zeise's salt was published. [43][44] Evidence for σ donation and the π-backbonding interaction was supported by the facts that the hydrogens on the ethylene are bent away from the normal C=C-H plane by a dihedral angle of 32.5 o , 45 and that the C=C bond length of coordinated ethylene (1.375 Ȧ) 46 is longer than that of free ethylene (1.337 Ȧ According to the DCD description, η 2 -alkenes are considered to be two electron neutral ligands which normally bond side-on to a metal atom with both carbon atoms of the double bond equidistant from the metal with the other groups on the alkene approximately perpendicular to the plane of the metal atom and the two carbon atoms.…”

“…Qualitative interpretations of the model suggest that π-bonding dominates the interaction between electron-rich metals and olefins implying, for example, that halogenated olefins would bind to metals stronger than ethylene because they are better π acceptors of electron density. 1,[48][49] [Fe(CO) 4 (η 2 -C 2 X 4 )] (X = H, F, Cl), which provide a rational for the inadequacy of the DCD description. 22,24 Further studies by Schlappi and Cedeño also found that the bonding of the perhalogenated olefins to Ni(PH 3 ) 2 (CO) follow a trend very similar to the one shown in Tolman's study and confirmed his presumption that the reason for the inadequacy of the DCD picture of metal-olefin bonding was due to the reorganization that occurs in the olefin due to rehybridization from sp 2 to sp 3 upon complex formation.…”