The oxidative addition of allylic halides to (C5R5)RuL2X (R = H, Me; L = CO, PPh3) gave new Ru-(IV)–η3-allyl complexes, (C5R5)RuX2(η3-allyl). An X-ray structure determination was carried out on (C5Me5)RuBr2(η3-C3H5), indicating a pseudo-piano-stool structure having two Br atoms and two terminal carbons of the endo-η3-allyl ligand located at the basal positions. There is a crystal mirror plane bisecting the pentamethylcyclopentadienyl and the π-allyl ligands. Crystal data: orthorhombic, space group P212121, a = 11.738 (1) Å, b = 13.367 (7) Å, c = 9.383 (1) Å, Z = 4, data refined to R = 0.0695. Its1H and13C NMR spectra showed symmetric allyl signals, supporting that the above-described piano-stool structure is maintained even in solution. The oxidative addition of allylic halides to (C5R5)Ru(CO)2X is reversible, since the reductive elimination of allylic halides from the Ru(IV)-allyl complexes proceeded under a CO atmosphere to re-form the Ru(II)–carbonyl compounds. (C5H5)RuCl2(η3-η3C3H5) reacted with DMSO to give the corresponding σ-allyl complex. This result indicates the involvement of σ-π interconversion in the reversible oxidative addition of allylic halides to the Ru(II) precursors. The extrusion of allylic halides from the Ru(IV)-allyl complexes also occurred in hot aromatic hydrocarbons to form cationic Ru(II) complexes, [(C5H5)Ru(arene)]+X-.
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