Four separate fluxional processes in (η6-cyclo-octatetraene)(η4-cyclo-octa-1,5-diene)osmium. A novel example of a metal [1,5] shift on a π-co-ordinated polyene ligand

Abstract

The ¹³C n.m.r. spectrum of Os(η⁶-C₈H₈)(η⁴-C₈H₁₂) in [²H₈]toluene at -93 °C shows that the molecule possesses no symmetry, and a low energy twitch occurs with ΔG^‡ 8.9 kcal mol^–1 producing an apparent plane of symmetry; at higher temperature, the cyclo-octatetraene and cyclo-octadiene ligands mutually rotate, with ΔG^‡ca. 11.5 kcal mol^–1, exchange commencing at room temperature on the cyclo-octatetraene ring by a novel [1,5] shift mechanism with ΔG^‡= 16.5 kcal mol^–1 and at slightly higher temperature a general shift of the metal around the cyclo-octatetraene ring occuring with ΔG^‡ca. 19.5 kcal mol^–1(1 cal = 4.184 J).