Nuclear magnetic resonance in transition-metal complexes

Abstract

Ligand nuclei chemical shifts produced by the formation of a metal–ligand bond have been classified in three classes; one with a shift of the complex resonance which is to high field of the free ligand resonance and approximately invariant to the metal and stereochemistry; a second class with a low-field shift of the complex resonance which varies roughly inversely as the energies of low-lying, excited electronic states; and a third with a high- or low-field shift of the uncomplexed ligand resonance, also showing the inverse excitation energy-dependence. A discussion suggests that these classes arise from the combination of two electronic effects, one from a localised metal-to-ligand electron-pair bond and the other from a molecularly delocalised metal–ligand interaction. Both effects come within the paramagnetic term of Ramsey's expression for the nuclear screening constant.