Cationic (η ³ -allylic)(η ⁴ -diene)-palladium and -platinum complexes

The syntheses, and the temperature-dependent 1H and 13C n.m.r. spectra of the complexes [Pd(η3-all)(η4-diene)]-[PF6][all = C3H5, 1 -MeC3H4, 2-MeC3 H4, or 2-PhC3H4, diene = cycle-octatetraene (cot) : all = 2-MeC3H4, diene = cycle-octa-1.5-diene (cod), hexamethylbicyclohexa[2.2.0]diene (hmdb), hexa-1,5-diene, or cyclo-heptatriene], [Pt(η3-2-MeC3H4)Cl]2, and [Pt(η3-2-MeC3H4)(η4-diene)][PF6][diene = cot or cod] are reported. The platinum complexes are rigid at low temperatures but undergo η3→η1→η3 exchange on heating. The palladium complexes exhibit different behaviour; for the cot complexes the observations are consistent with a higher temperature exchange process involving solvent, [Pd(all)(cot)]++ s ⇌ cot +[Pd(all)(s)2]+, and a lower energy process which could not be frozen out and which is interpreted in terms of a solvated five-co-ordinate intermediate undergoing pseudorotation in which the two halves of the cot are made equivalent. The η-arene complexes [M(2-MeC3H4)(hmb)][PF6](M = Pd or Pt, hmb = hexamethylbenzene) have been prepared and their structures are discussed on the basis of their n.m.r. spectra. A stability sequence towards solvation by acetone for the palladium complexes is, cot ∼ cod > hmdb > hexa-1,5 diene ∼ cht ∼ hmb; the Pt complexes are less stable than their Pd analogues.