C-C σ-bond activation of cyclopropene by RuCl2(PPh 3)2 and IrCl(CO)(PMe3)2 was theoretically investigated. The activation barrier and the reaction energy calculated here indicate that the reaction occurs more easily by the ruthenium complex than by the iridium complex, which is consistent with the experimental results that the Ru-vinylcarbene species is formed but the Ir-vinylcarbene is not. The valence bond analysis of the CASSCF wave function disclosed that the Ir-vinylcarbene bond is significantly weaker than that of the Ru complex because the d6 square pyramidal complex of Ru is more favorable than the d8 complex of Ir for this bonding interaction. In both the Ru and Ir systems, a precursor complex is formed by coordination of the C 1=C2 double bond of cyclopropene with the metal center, where C1 and C2 are sp2 carbons and C 3 is an sp3 carbon. In the transition state, one C-C single bond (named C1-C3) is almost broken, but the M-C1 bond (M = Ru or Ir) and another C-C single bond (named C 2-C3) are becoming stronger. When moving from the transition state to a metal-vinylcarbene product, the C1=C 2 double bond changes to the C1-C2 single bond with the concomitant change of the C2-C3 single bond to the C2=C3 double bond. To induce these bond formation and bond breaking processes, the valence state of the metal center must change in the reaction. The promotion energy to the valence state becomes smaller in the ruthenium reaction system when going from the reactant to the product but becomes considerably larger in the iridium reaction system. This is the reason that the C-C σ-bond cleavage of cyclopropene occurs more easily in the ruthenium complex than in the iridium complex. The difference in promotion energy between the ruthenium and iridium systems is reasonably interpreted in terms of d-d orbital splitting by ligand-field and d electron number.
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