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.
Density functional theory (DFT) studies have been performed in the title complexes to determine the effect of electron-withdrawing halogens around the CdC bond on the metal-olefin bond dissociation energy. Calculations indicate that the nickel-olefin bond dissociation energy would be nearly independent of the number of electron-withdrawing elements around the double bond. The results are explained in terms of electronic, steric, and reorganizational effects that derive from the olefin-metal interaction and are compared to computational and experimental results for related complexes, which show similar metal-olefin bond strength behavior. It has been found that reorganizational effects in both the olefin and Ni(PH 3 ) 2 (CO) play a determining role in the overall bond dissociation energy trend.
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