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 f ragment 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-withdrawi ng (Cl, CN, COOMe, SO(2)Ph) and electron-donating (Me, Ph) substituent s at either the terminal or central carbon of the allyl framework. The relative ability of a substituted allyl Ligand to coordinate to the P d(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 with drawing substituent coordinated to the Pd-Pd fragment more strongly th an that bearing the less withdrawing substituent. The 1-methyl, 1-phen yl, and 1-chloroallyl ligands on the Pd-Pd bond exist in an anti confi guration 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 = CH2 C(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 r esults revealed a unique geometrical feature of the bridging allyl lig and; the dihedral angle between the allyl plane and the approximate co ordination plane (M-M-X plane) is smaller than 90 degrees. Ab initio M O calculations on the model Pd-2(mu-CH2CHCH2)(mu-Br)(PH3)(2) were perf ormed to reveal important aspects of the bonding nature; the coordinat e bond of the mu-allyl ligand involved not only donation from the ally l 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 th e occupied d sigma-d sigma and d pi-d pi bonding combinations of the P d-2(mu-Br)(PH3)(2) unit to the allyl pi orbital. The latter is not ob served in the mononuclear palladium(II) allyl complex but is found as a characteristic interaction in the dinuclear mu-allyl complex. The ab ove-mentioned unique geometrical and stability features of the dinucle ar mu-allyl complexes arise from the back-bonding interaction.