Critical analysis of the limitations of Bleaney's theory of magnetic anisotropy in paramagnetic lanthanide coordination complexes

The origins of the breakdown of Bleaney's theory of magnetic anisotropy are described, based on an analysis of eleven different complexes of the second half of the 4f elements that form isostructural series. An examination of the chemical shift and relaxation rate behaviour of resonances located at least four bonds away from the paramagnetic centre was undertaken, and correlated to theoretical predictions. The key limitations relate to comparability of ligand field splitting with spin–orbit coupling, variation in the position of the principal magnetic axis between Ln complexes and the importance of multipolar terms in describing lanthanide ligand field interactions.