Structural preferences in eta(2)-alkenyl transition-metal complexes ML5(eta(2)-alkenyl) and MCpL2(eta(2)-alkenyl)

Density functional molecular orbital calculations have been used to study t he preference for coplanarity of the eta(2)-alkenyl and one of the metal-li gand bonds in transition-metal eta(2)-alkenyl complexes. The metal-eta(2)-a lkenyl bonding in these complexes involves the Dewar-Chatt-Duncanson type o f interactions (the donation of eta(2)-alkenyl pi electrons to the metal ce nter and the metal(d)-eta(2)-alkenyl(pi*) back-donation) and metal(d)-C-alp ha(p) pi bonding. The Dewar-Chatt-Duncanson type of interactions is found t o be the dominant factor in determining the structural preference while the metal-C-alpha pi bonding strength remains approximately constant with resp ect to the eta(2)-alkenyl rotation. When the C-alpha-C-beta vector of the e ta(2)-alkenyl ligand eclipses (is coplanar with) one of the metal-ligand bo nds, the metal-eta(2)-alkenyl interactions are the strongest because the mi xing of metal d-p orbitals, resulting from the metal-ligand distortion, enh ances the metal(d)-eta(2)-alkenyl(pi*) back-donation. For eta(2)-alkenyl co mplexes containing a Cp ligand, the steric effect of the bulky Cp ligand is also operative although the electronic factors are still dominant in deter mining the structural preference.