The Gadolinium Break

Although both Klemm and Noddack stuck to their respective diad and triad classifications of the lanthanides until the end of the discussion and later, it became, however, clear that a break do exist around the gadolinium region (not an experimental error, as many have supposed at that time), and that the gadolinium break is observed either (a) very weakly (or not at all) for some properties or (b) very pronounced for other properties of the lanthanide series. 

Usually, the plot of Goldschmidt s ionic radii produced no inflection around the gadolinium region although Bommer 12) later supported Klemm s diad theory by plotting the cell constant (a) for the C-type lanthanide oxides. To support his diad theory Klemm has (13) also pointed out that while La(III) and Lu(III) possess empty (4 f°) and completely filled (4/14)/-shell respectively, Gd(III) has a half-filled shell (4/7), and that usually a break is observed around the half-filled shell. He plotted (13) the ionization potentials for the M - M+ reaction for the series B—Ne and Al-Ar and showed that a break does exist in the N—0 and P—S region. In Fig. 2 several plots are made using the newly acquired data (14). 

Thus the idea of gadolinium break and its association with the half-filled 4/-shell started to crystallized out around 1930. 

The plots of the ionic radii, molar volume of the octahydrated sulphates, the basicity of the hydroxides, the predicted ionization energy and the hydration energies by Klemm (10) for the lanthanides showed (10,12) so weak a discontinuity near gadolinium that it raised the question whether or not the experimental errors in measuring these quantities are showing up as the break. To add some weight, Moeller and Kremers (15) in their review on the basic characteristics of Sc, Y and the lanthanides remarked (p. 119)  

As more and more experimental results on the formation constants and the thermodynamic properties were obtained, it became positively clear that the region of the gadolinium break is not confined in gadolinium and that it may start as early as samarium and may extend upto dysprosium. This is exemplified in some representative plots of the formation constants in Fig. 3. It is obvious that in some cases severe gadolinium break has been observed. 

---

The stability constants show the same trend as with acetate having an extended region (Eu—Ho) of the gadolinium break. 

It will be noticed (Table 37) that the entropy of formation of the am oacid complexes are quite large and again on the basis of A/Sj values the complexes could be divided into two groups. Thus, at present, there seems to be no simple explanation of the gadolinium break. 

The trend of log kn in glyoxylate complexes is more aimilar to that of the glycolate system than the acetate system. The gadolinium break... 

In the case of a majority of ligands, the gadolinium complex is less stable than one would expect based on the simple electrostatic model. This has been known as the gadolinium break which cannot be explained in terms of steric hindrance since this anomaly is present even with ligands which offer no steric hindrance. 

Danilov and co-workers (20) studied the extraction of lanthanides with a,a -dialkylcarboxylic and hexanoic acids in n-heptane. The extracted species is invariably LnA3(HA)3. As is obvious from Table II, the gadolinium break is observed in the extraction constants. 

The group of ligands for which log vs. the atomic number plot usually results in sinus curves. The gadolinium break is most pronounced. Example acetate, isobutyrate, propionate etc. 

In both cases (4) and (5) the gadolinium break seems to start earlier than Gd. The log Kj value for the Eu-complex is lower than that for Sm (Fig. 3). 

We have certainly noticed in the previous discussion on the gadolinium break that the formation constants are not a simple function of the atomic numbers, and that the plots of the thermodynamic quantities (Fig. 3, 5) leave room for further subdivision of the diad series of Klemm. Actually Peppard etal. (41) during their long time involvement in the liquid-liquid extraction studies of the lanthanides and the actinides (41 —43) observed that the diad series of Klemm could be further subdivided into two subgroups each, giving rise to what they termed as a tetrad series. They summarized (43) their findings as ... 

Persson, I., D Angelo, P., De Panfilis, S., Sandstrom, M., and Eriksson, L. (2008) Hydration of lanthanoid(III) ions in aqueous solution and crystalline hydrates studied by EXAFS spectroscopy and crystallography the myth of the gadolinium break. Chem. Eur. J, 14, 3056-3066. 

---