Comparison of crystal field dependent and independent methods to analyse lanthanide induced NMR shifts in axially symmetric complexes. Part II: Systems with a C₄ symmetry axis

Analysis of the LIS data for several series of Ln³⁺ complexes of C₄ symmetry in terms of structural changes, crystal-field effects and/or variation of hyperfine constants along the lanthanide series was undertaken using a combination of the two-nuclei and three-nuclei techniques together with the classical one-nucleus technique. Isostructurality of whole series of complexes, with changes of the F_i, and B₀ ² parameters, was clearly defined for the complexes of L⁵ by the combination of the two first methods. Small changes, involving the three F_i, G_i and B₀² parameters, are observed for the series of complexes of L¹-L⁴, using the three data plotting methods. Some of the plots according to the two- and three-nuclei methods are accidentally linear, without necessarily implying isostructurality of the complexes, as they involve parameters, which may be insensitive to any small structural changes occurring in these systems. These parameter variations could result from a magnification, by the present graphical analysis, of the breaks expected from the gradual structural changes along the series due to the lanthanide contraction. The α and β parameters of the three-nuclei method are not diagnostic of the type of structures the complexes have in solution, due to their very indirect dependence on the geometric factors.