Synthesis and Reactivity of Cationic (Allyl)(arene)nickel(II) and (Allyl)(arene)palladium(II) Complexes

[(2-R-allyl)M(arene)]+ complexes (M = Pd, Ni; R = H, CH3, Cl; arene = mesitylene, hexamethylbenzene) have been synthesized via halide abstraction from the corresponding allyl halide dimers, [(allyl)MX]2, using either AgSbF6 in the case of M = Pd or NaB(Arf)4 (Arf = 3,5-(CF3)2C6H3) in the case of M = Ni. The [(allyl)Ni(mesitylene)]+ and [(2-methallyl)Ni(hexamethylbenzene)]+ salts have been characterized by single-crystal X-ray diffraction. The arene ligands in the Pd species are highly labile. The mesitylene ligand in the [(2-R-allyl)Pd(mesitylene)]+ complexes is rapidly displaced at temperatures as low as −120 °C by olefins and alkynes (ethylene, tert-butylethylene, cyclopentene, cyclohexene, cyclooctene, 2-butyne) to yield the bis-olefin or bis-alkyne complexes, which have been characterized by NMR spectroscopy. [(allyl)Pd(mesitylene)]+ undergoes rapid degenerate exchange with free mesitylene at low temperatures (ΔG⧧ = 10.2 kcal/mol). The arene ligand of the Ni complexes is less labile. Displacement of mesitylene from [(allyl)Ni(mesitylene)]+ by excess diethyl ether at 25 °C yields [(allyl)Ni(Et2O)2]+. Reaction of the [(2-R-allyl)Ni(mesitylene)]+ complexes (R = H, CH3) with α-olefins at 25 °C yields new allyl complexes plus propene (when R = H) or isobutylene (when R = CH3). A mechanism involving intramolecular hydrogen migration is proposed to account for these transformations.