2000 Alcan Award Lecture Adventures in organometallic NMR: steric restraints, slowed rotations, and skeletal rearrangements

The structures and NMR fluxionality of a wide variety of organometallic complexes (including tetrahedral cluster cations and systems of the type (C n R n )ML 3 , where R = Et, Ph) are discussed. Barriers to tripodal and peripheral substituent rotations are reported, and the relevance of correlated rotations to molecular machines is outlined. Haptotropic shifts in (cyclopenta[def]phenanthrenyl)ML n and (cyclopenta[l]phenanthrenyl)ML n complexes are rationalized in terms of the aromatic character of the transition states. Likewise, the barriers to silatropic shifts in polyindenylsilanes can be correlated with the aromaticity of the intermediate isoindenes. Finally, the use of cobalt clusters to control cyclohexane ring conformations is described.Key words: NMR ring currents, fluxionality, correlated rotations, molecular rearrangements, haptotropic shifts.