New insights into structures, stability, and bonding of mu-allyl ligands coordinated with Pd-Pd and Pd-Pt fragments

A series of complexes of mu-allyl ligands coordinated with the Pd-Pd fragment Pd-2(mu-allyl)-(mu-Cl)(PPh(3))(2) were prepared from reactions of the corresponding allylpalladium(II) chlorides with Pd(C2H4)(PPh(3))(2). The mu-allyl ligands employed contained both electron-withdrawing (Cl, CN, COOMe, SO(2)Ph) and electron-donating (Me, Ph) substituents at either the terminal or central carbon of the allyl framework. The relative ability of a substituted allyl Ligand to coordinate to the Pd(I)-Pd(I) fragment vs the mononuclear Pd(II) fragment was determined by means of an allyl ligand exchange equilibrium between the dinuclear and the mononuclear fragments. The allyl ligand bearing the more withdrawing substituent coordinated to the Pd-Pd fragment more strongly than that bearing the less withdrawing substituent. The 1-methyl, 1-phenyl, and 1-chloroallyl ligands on the Pd-Pd bond exist in an anti configuration almost exclusively. X-ray structure determinations of some of the mu-allyl Pd-Pd and Pd-Pt complexes, PdM(mu-allyl)(mu-X)(PPh(3))(2) (11, M = Pt, allyl = CH2C(COOMe)CH2, X = Pr; 12, M = Pd, allyl = CH2C(COOMe)CH2, X = Pr; 13, M = Pd, allyl = CH2CHCH(COOMe), X = SPh) were carried out. The crystal of 11 was isomorphous with that of 12. The results revealed a unique geometrical feature of the bridging allyl ligand; the dihedral angle between the allyl plane and the approximate coordination plane (M-M-X plane) is smaller than 90 degrees. Ab initio MO calculations on the model Pd-2(mu-CH2CHCH2)(mu-Br)(PH3)(2) were performed to reveal important aspects of the bonding nature; the coordinate bond of the mu-allyl ligand involved not only donation from the allyl nonbonding pi orbital to the unoccupied d sigma-d sigma antibonding orbital of the Pd-2(mu-Br)(PH3)(2) unit but also back-donation from the occupied d sigma-d sigma and d pi-d pi bonding combinations of the Pd-2(mu-Br)(PH3)(2) unit to the allyl pi* orbital. The latter is not observed in the mononuclear palladium(II) allyl complex but is found as a characteristic interaction in the dinuclear mu-allyl complex. The above-mentioned unique geometrical and stability features of the dinuclear mu-allyl complexes arise from the back-bonding interaction.