Fluxionality and lability in rhenium 4'-hydroxyterpyridine complexes: evidence for an associative mechanism and correlated fluxionality and lability.

The complexes [ReX(CO)(3)(N,N-eta(2)-4'-hydroxy-2-2',6'-2''-terpyridine)] (X = Cl,Br,I) have been synthesised and their ligand exchange reactions and fluxionality of the terpyridine unit studied. The halides are far more labile in these species than in analogous bipyridines, and it is proposed that the ligand fluxionality is involved in this reactivity. Structural studies of the family are reported along with spectroscopic studies including variable temperature NMR which is used to demonstrate a negative entropy of activation for the fluxional process. Synthesis of an analogue which is incapable of fluxional behaviour confirms the link between fluxionality and lability in these complexes.