Addition of fluoride to aqueous solutions of lanthanide complexes of DTMA results in the formation of ternary complexes of the form [F·Ln·DTMA](2+) in which an axial solvent molecule is displaced by fluoride. [F·Ln·DTMA](2+) and [H2O·Ln·DTMA](3+) are in exchange on a timescale of around 1 s. Dramatic changes are observed in both the NMR and luminescence spectra of the complexes: these are consistent with a change in the nature of the magnetic anisotropy at the paramagnetic lanthanide centre, itself arising from a change in the local crystal field. Study of paramagnetic lanthanide complexes with anisotropic electronic distributions reveals that, upon replacing water with fluoride, there is an inversion of the sign, and a significant reduction in the magnitude, of the crystal field term that defines the nature of the pseudocontact shift.
