Structural Preferences in η2-Alkenyl Transition-Metal Complexes ML5(η2-alkenyl) and MCpL2(η2-alkenyl)

Abstract

Density functional molecular orbital calculations have been used to study the preference for coplanarity of the η2-alkenyl and one of the metal−ligand bonds in transition-metal η2-alkenyl complexes. The metal−η2-alkenyl bonding in these complexes involves the Dewar−Chatt−Duncanson type of interactions (the donation of η2-alkenyl π electrons to the metal center and the metal(d)−η2-alkenyl(π*) back-donation) and metal(d)−Cα(p) π bonding. The Dewar−Chatt−Duncanson type of interactions is found to be the dominant factor in determining the structural preference while the metal−Cα π bonding strength remains approximately constant with respect to the η2-alkenyl rotation. When the Cα−Cβ vector of the η2-alkenyl ligand eclipses (is coplanar with) one of the metal−ligand bonds, the metal−η2-alkenyl interactions are the strongest because the mixing of metal d−p orbitals, resulting from the metal−ligand distortion, enhances the metal(d)−η2-alkenyl(π*) back-donation. For η2-alkenyl complexes containing a Cp ligand, the steric effect of the bulky Cp ligand is also operative although the electronic factors are still dominant in determining the structural preference.