Applications of Nuclear Magnetic Resonance to the Study of Organometallic Compounds

Abstract

Publisher Summary Quantum mechanical time scale of the nuclear magnetic resonance (NMR) experiment helps to study certain time-dependent phenomena that are not generally accessible to the other branches of spectroscopy. Both molecular motion and chemical exchange may affect the appearance of NMR spectra. This unique characteristic is finding significant application in the study of organometallic compounds. This chapter describes the use of NMR as an investigative tool and discusses the chemical shift and spin–spin coupling. The calculation of chemical shifts is closely related to the calculation of diamagnetic susceptibilities, whereas the calculation of spin–spin coupling constants can be achieved using the valence-bond approximation. A characteristic property of some organometallic systems is the exchange of alkyl groups from one metal to another. Exchange of alkyl groups may take place between different metals or the same metal in two different oxidation states that have two characteristic resonances. In alkenyl derivatives and olefin π complexes, a phenomenon in addition to metal–ligand interchange is observed––that is, the exchange of metal-to-carbon bond from one site to another in the same ligand. This causes the averaging of chemical shifts for protons in different magnetic environments in the frozen molecule, but it preserves metal–proton spin–spin coupling satellites although the observed splittings are the average of the extreme values in the frozen molecule. An intramolecular exchange similar to that noted for alkenyl derivatives is also noted for some olefin complexes.