ChemInform Abstract: FLUXIONAL AND NONRIGID BEHAVIOR OF TRANSITION METAL ORGANOMETALLIC Π‐COMPLEXES

Abstract

Publisher Summary The chapter discusses major developments in the understanding of nonrigidity in π-complexes . The stereochemistry of products derived from reactions of coordinated olefins and the stereochemistry of polymers formed in reactions catalyzed by transition metals are ultimately determined by the conformational stability of π-complexed intermediates. The interconversions rates of these isomers and the thermodynamic preferences of olefin–metal conformations should explain observed product distributions and provide a rational basis for catalyst design. Molecules that undergo intermolecular rearrangements at rates that influence nuclear magnetic resonance (NMR) line shapes in accessible temperature ranges are termed “stereochemically nonrigid.” Nonrigid molecules, of which all observable interconnecting species are chemically and structurally equivalent, are designated as fluxional. The study of nonrigid systems has closely followed the technical advance of NMR instrumentation. The rapid advances in 13 C spectroscopy have made previously undetectable carbonyl rearrangements, conformational interconversions, and fluxionality accessible. This chapter concentrates on recent developments in technique that are likely to be used more extensively, potential problems in interpretation of dynamic NMR spectra, and observations that establish general patterns of rearrangement or conflict with currently accepted mechanisms. Stereochemically nonrigidity can be interpreted in these systems in terms of variations in bond strength with orientation of the olefin. Rotation of the allyl group, in which syn–anti interconversions does not occur, is now known in many instances for complexes with coordination numbers greater than four. NMR spectra consistent with allyl rotation in most square palladium complexes have been shown to result from ligand exchange.