Geometries, Bonding Nature, and Relative Stabilities of Dinuclear Palladium(I) π-Allyl and Mononuclear Palladium(II) π-Allyl Complexes. A Theoretical Study

MP2 optimization of Pd 2 (μ-Br)(μ-C 3 H 5 )(PH 3 ) 2 ( 1 ) and PdCl(η 3 -C 3 H 5 )(PH 3 ) ( 2 ) well reproduces geometrical characteristics of these complexes. For instance, the optimized dihedral angle between π-allyl and Pd 2 Br planes is 83° in 1, and the dihedral angle between π-allyl and PdCl(PH 3 ) planes is 115° in 2 . These optimized values agree well with the experimental results (the deviation is less than 1°). Although the π-allyl coordinate bond of 2 is mainly formed by donation from the π-allyl nonbonding π (nπ) orbital to the unoccupied d orbital of Pd, the μ-allyl coordinate bond of 1 is formed by back-donation from the Pd−Pd d σ bonding orbital to the π-allyl π* orbital and donation from the π-allyl nπ orbital to the Pd−Pd d σ antibonding orbital. To maximize these two interactions, two palladium atoms take their positions under the terminal carbon atoms of μ-allyl group. In addition to these interactions, the back-donating interaction between the μ-allyl π* and the Pd−Pd d π bonding orbitals participates in the μ-allyl coordination. The dihedral angle θ of 1 decreases to 83° to enhance the above-mentioned two back-bonding interactions. Introduction of the electron-withdrawing CN group to π-allyl enhances the stability of 1 and decreases the dihedral angle θ. However, introduction of the electron-releasing CH 3 group to π-allyl little changes the dihedral angle of 1 but enhances the stability of 2 . These substituent effects, as well as the difference in the dihedral angle between 1 and 2, are clearly interpreted in terms of the coordinate bonding nature.