Applications of nuclear quadrupole resonance spectroscopy to metal complexes

Abstract

Nuclear quadrupole resonance (NQR) spectroscopy provides information on the ground state charge distribution about a quadrupolar nucleus, because the NQR spectrum is determined by the components of the electric field gradient (EFG) tensor. Metal complexes may be studied via NQR methods by observation of the spectra of quadrupolar metal nuclei. Interpretation of the data in terms of metal-ligand interactions is often difficult because of the large number of counterbalancing contributions. The NQR spectra of ligand nuclei can often provide a more direct access to details of the metal-ligand bonding. It has recently become possible to study the 14N NQR spectra of coordinated nitrogen, and a substantial body of data for several nitrogen-containing ligands has been obtained. These NQR results can be interpreted in terms of the “coordinated nitrogen” model, an adaptation of the Townes-Dailey model. Using this approach it is possible to obtain an estimate of the population of the nitrogen donor orbital directed toward the metal ion or other Lewis acid. Thus the NQR experiment provides information on the extent of charge transfer from the ligand to the Lewis acid center in the ground electronic state.