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.