Metal-olefin interactions in M(CO)5(cycloolefin) (M = Cr, Mo, W; Cycloolefin = cyclopropene to cyclooctene):: Strain relief and metal-olefin bond strength

Density functional theory calculations on the title compounds indicate that metal-olefin bond strengths follow the trend for cyclic olefin strain energies. It was found, however, that the proportionality between metal-olefin bond energy and strain energy is not evenly distributed throughout the olefin series. For instance, cyclopropene and cyclobutene are expected to bind to the metal much more weakly than would be anticipated on the basis of their strain energies. A bond energy decomposition analysis reveals that the metal-olefin interaction is responsible for strain relief in the cycloolefins by means of the rehybridization of the olefinic carbons. However, the geometrical changes accompanying this rehybridization, namely C=C elongation and olefin pyramidalization, involve an energetic cost that is paid at the expense of the bonding interaction energy. Nonpyramidalized strained olefins such as cyclopropene and cyclobutene undergo large conformational changes, to the detriment of their large attractive interaction energies. It was found that a cyclic olefin that is already deformed, such as trans-cyclooctene, interacts strongly with a metal to relieve strain but does not suffer much energy-costly reorganizations. This, thus, constitutes an energy benefit to the metal-trans-cyclooctene bond strength.