'Lanthanide contraction' in [Ln(BTP)3](CF3SO3)3 complexes

The FTIR spectra of the complexes of lanthanides (Ln = La − Lu, except for the radioactive Pm) with 2,6-bis(5,6-dipropyl-1,2,4-triazin-3-yl)pyridine have been measured in both the mid- and far-IR ranges. The spectra revealed some characteristic bands shifting towards higher wavenumbers with increasing lanthanide atomic number. X-ray diffraction results on several (La, Pr, Nd, Sm, Eu, Tm and Yb) complexes revealed decreasing Ln–ligand distances in the same direction. DFT calculations using the M062X exchange–correlation functional have been performed on selected model complexes in order to uncover the ‘lanthanide contraction’ phenomenon and to assist the interpretation of the FTIR spectra. The stronger bonding due to ‘lanthanide contraction’ is supported by the computed charge transfer interactions. The vibrational study confirmed that the blueshifting bands can be attributed to significant Ln–ligand vibrations.