The replacement of one of the acetate pendant arms with a 2-methylpyridine-N-oxide group in the molecule of H(4)dota significantly alters the coordination properties of the ligand in Ln(III) complexes. The structural properties of the complexes are investigated both in solution and in the solid state. The variable-temperature H-1 NMR spectra of Nd-III, Eu-III, and Yb-III complexes show that the twisted-square-antiprismatic (TSA) isomer is strongly destabilized and suppressed in solution and the complexes exist mostly as the square-antiprismatic (SA) isomers (98% for Eu-III at -35 degrees C). The exchange between the TSA and SA isomers is fast at room temperature compared to that of the NMR time scale. The flexibility of the six-membered chelate ring formed by coordination of the 2-methylpyridine-N-oxide group to the central ion allows two orientations of this pendant arm relative to the acetate arms: syn-SA (pyridine in the direction of the acetates) and anti-SA (pyridine opposite to the acetates). The syn-SA form was found in the X-ray structure of the Nd-III complex; the anti-SA forms were found in the structures of Dy-III, Tm-III, and Yb-III complexes. The UV-vis and H-1 NMR spectra of the Eu-III complex suggest that both forms are in dynamic equilibrium in solution. A derivatization of the pyridine-N-oxide group with a carboxylic group in the 4 position has no significant effect on the properties of the Ln(III) complexes.