The combination of one contact and three pseudo-contact contributions to the NMR hyperfine paramagnetic shift of each proton in the triple-stranded helicates [Ln₃(L1)₃]⁹⁺(Ln = Ce–Yb except Pm, Gd) produce intractable ¹H NMR spectra whose assignment is limited by the large electronic contribution to the nuclear relaxation processes. The detailed analysis of the NMR spectra for the diamagnetic complexes [Ln₃(L1)₃]⁹⁺(Ln = La, Lu, Y) shows that the triple-helical structure found in the solid state is maintained in solution. Extension of the classical one-nucleus crystal-field dependent model-free method for paramagnetic D₃-symmetrical homotrimetallic lanthanide complexes possessing two different metallic sites (i.e. two second-rank crystal-field parameters: B20^central and B20^terminal) allows (i) the complete interpretation of the paramagnetic signals for Ln = Ce–Yb and (ii) the detection of a concomitant abrupt change of the contact terms F_i and of the pseudo-contact terms S_i=B20^centralG1i+B20^terminal(G2i+G3i) occurring near the middle of the lanthanide series. The derivation and application of a novel three-nuclei crystal-field independent method eventually demonstrates that the helicates [Ln₃(L1)₃]⁹⁺ adopt a single D₃-symmetrical structure along the complete lanthanide series in solution, which ascribes the discontinuity observed for S_i to a concomitant decrease of the two crystal-field parameters. Comparison with structural models is limited by the extreme sensitivity of the structural factors C_ikl and D_ikl to minor geometrical variations affecting the wrapping of the ligand strands, but calculations of the geometrical factors Gmi(m= 1–3) for [Ln₃(L1)₃]⁹⁺ in solution confirm the formation of a regular triple-helical structure. Generalization of this novel three-nuclei method for addressing the solution structure of rhombic lanthanide complexes is discussed.

Variable-temperature ¹H and ¹³C NMR measurements of the D₃-symmetrical triple-helical complexes [Ln(L1-2H)₃]^3- (L1 = pyridine-2,6-dicarboxylic acid; Ln = La−Lu) show evidence of dynamic intermolecular ligand-exchange processes whose activation energies depend on the size of the metal ion. At 298 K, the use of diastereotopic probes in [Ln(L3-2H)₃]^3- (L3 = 4-ethyl-pyridine-2,6-dicarboxylic acid) shows that fast intramolecular P ↔ M interconversion between the helical enantiomers occurs on the NMR time scale. Detailed analyses of the paramagnetic NMR hyperfine shifts according to crystal-field independent techniques demonstrate the existence of two different helical structures, one for large lanthanides (Ln = La−Eu) and one for small lanthanides (Ln = Tb−Lu), in complete contrast with the isostructurality proposed 25 years ago. A careful reconsideration of the original crystal-field-dependent analysis shows that an abrupt variation of the axial crystal-field parameter A₂⁰²>^ parallels the structural change leading to some accidental compensation effects that prevent the detection of structural variations according to the classical one-nucleus method. Crystal structures in the solid state and density functional theory calculations in the gas phase provide structural models that rationalize the paramagnetic NMR data. A regular triple-helical structure is found for small lanthanides (Ln = Tb−Lu) in which the terdentate chelating ligands are rigidly tricoordinated to the metals. A flexible and distorted structure is evidenced for Ln = La−Eu in which the central pyridine rings interact poorly with the metal ion. The origin of the simultaneous variation of structural parameters and crystal-field and hyperfine constants near the middle of the lanthanide series is discussed together with the use of crystal-field-independent techniques for the interpretation of paramagnetic NMR spectra in axial lanthanide complexes.