Isomerism in the Terbium(III) Trifluoroacetate Trihydrate Dimer

Precision X‐ray diffraction analysis of a perfect single crystal showed that the molecule of terbium(III) trifluoroacetate trihydrate is a combination of two isomers (monomeric form), Tb(CF3COO)3(H2O)3 and Tb(CF3COO)2(CF3COOH)(H2O)2(OH) with a statistical weight of 0.5. Based on precision X‐ray diffraction analysis, the statistical composition of the compound resulting from neutralization of terbium(III) hydroxide with trifluoroacetic acid is Tb(CF3COO)2.5(CF3COOH)0.5(H2O)2.5(OH)0.5. The presence of the CF3COOH molecule and the OH- group is also confirmed by IR and 1H NMR spectra, respectively. The presence of 18 rather than the anticipated nine Stark components of the 5D4→7F0 transition of the Tb3+ ion can be explained in terms of both a static and a dynamic model. In the static case, the compound can be represented as a combination of equal fractions of two isomers; hence the doubled number of Stark components. In this case, the molecule of the dimer remains statistically centrosymmetric. In the dynamic version, migration of a proton across the dimer from one of the water molecules to the nearest monodentate anion CF3COO- and back leads to an electron density redistribution in the vicinity of each Tb3+ ion within the dimer, the crystallographic equivalence of their positions being preserved. However, X‐ray structure analysis cannot distinguish between these two cases. In any case, the observed isomerism in terbium(III) trifluoroacetate trihydrate removes discrepancies between the data of X‐ray structural and luminescent analyses.