Other Lanthanoid Oxidation States
It is the variations in oxidation states from the +3 “norm,” which have provided the most interest [26]. When plotting out known oxidation states for elements, the question arises as to how extreme the conditions, or how unusual the ligands, that have been used in order to stabilize a specific oxidation state. Table 12.1 identifies those oxidation states that have a significant existence for simple compounds. In contradiction to this generality, praseodymium(V) is included (in parenthesis) as it is of intrinsic interest in the context of 4f0 configurations. It can be seen that the empty; half-filled; and filled f electron energy state plays a significant — but not exclusive role — in determining which other oxidation state(s) are feasible for a specific lanthanoid.
The 4f0 oxidation state is the expected state for lanthanum. For cerium, Ce(IV) is a common state, though highly oxidizing. In the next section, we will see that cerium(IV) has many similarities to members of Group 4. The oxidation of cerium(III) to cerium(IV) has relevance to geochemistry. In oxidizing waters, cerium is deposited as insoluble cerium(IV) oxide. This is one parameter by which the redox condition of ancient seas and oceans can be determined [29]. The cerium enrichment (as cerium(IV) oxide) compared with the other lanthanoids is known as the “positive” cerium anomaly.
Table 12.1 The 4f electron configurations corresponding to the common ion charges (adapted from Ref. [26])
Though an empty f shell would seem to be an obvious possibility for praseodymium, it seems that Pr(V) is, in fact, not a particularly favored oxidation state for the element. The only species obtained by the date of writing has been the ion [PrO2]+ under very low-temperature noble gas matrix isolation [28].
Several of the lanthanoids exhibit the +2 oxidation state [30], but it is only for europium and ytterbium that the +2 state is of major importance. In addition, terbium only “reluctantly” forms compounds in the +4 oxidation state [31], which is surprising considering it has a significantly lower 4th ionization energy [32].
As can be seen from Table 12.1, the progression: lanthanum(III); cerium(IV); and praseodymium(V) corresponds to the “empty-shell” 4f0 series, which would not be unexpected [33]. Similarly, ytterbium(II) and lutetium(III) correspond to the “full-shell” 4f14 configurations. The third isoelectronic set, europium(II); gadolinium(III); and terbium(IV) correspond to the “half-filled” 4f7 electron configuration. This so-called “stability of the half-filled shell” (see Chapter 2) is often discussed in the context of the main group elements [34] but it is also evident for the lanthanoids.
Restructuring the Lanthanoids
Though conventionally the lanthanoids are treated as a single continuous unit, attempts have been made to identify subcategories and possible rearrangements.
Figure 12.4 Cerium as a member of Group 4 in addition to the lanthanoids (modified from Ref. [35]).
Cerium as a Member of Group 4
Johansson et al. have singled out cerium on the basis of its +4 oxidation state to be better considered as a member of Group 4 [35]. This assignment is shown in Figure 12.4.
The Stacked Lanthanoid Arrangement
In geochemistry, the rare earth elements are classified as “light” or “heavy.” Of the lanthanoids, lanthanum to gadolinium are usually considered as “light” while terbium to lutetium are usually assigned as “heavy.” More of the “light” lanthanoids are in the Earth’s crust, while more of the “heavy” lanthanoids in the Earth’s mantle. This distinction comes about through the variation in ionic radii and hence the crystal structures in which the ion will fit. The trend of ionic radii results from the lanthanide contraction described earlier.
It was Ternström in 1976, who first proposed “stacking” the two half series on the basis of physical and chemical properties as: Ce–Gd and Tb–Lu [36]. Laing elaborated upon this concept, focusing upon chemical resemblances [21]. He noted (as stated earlier) that both europium and ytterbium form compounds in which they have a +2 oxidation state and therefore related to Group 2. Similarly, cerium commonly forms compounds in which it has the +4 oxidation state, and therefore should be associated with Group 4. Laing placed these three elements in their assigned groups, plus intervening lanthanum, gadolinium, and lutetium in Group 3. The other lanthanoids were then placed in order to complete each of the two subrows (Figure 12.5).
Though the early members fit, there is no evidence so far of any higher oxidation states for the later members of the lanthanoids. Laing subsequently changed his mind about the arrangement of the lanthanoids [37]. Instead, he devised a three-level sandwich that highlighted the +2 trio of [Ba–Eu–Yb], the +3 trio of [La–Gd–Lu], and the +4 trio of [Ce–Tb–Hf], with gadolinium being central, as shown in Figure 12.6.
Figure 12.5 The two-row lanthanoids according to Laing, with the surrounding elements shaded (modified from Ref. [21]).
Figure 12.6 Laing’s gadolinium-centered lanthanoid series (from Ref. [37]).
Dendrogram Restructuring
Horovitz and Sârbu used a cluster analysis to develop a similar double-row set [38]. The dendrogram that relied largely upon a variety of numerical values for each atom/ ion is shown here (Figure 12.7).
From the structure of the dendrogram, Horovitz and Sârbut devised a segment of the Periodic Table. This arrangement (Figure 12.8) largely matches Laing’s two-row lanthanoid pattern shown in Figure 12.5 (though Laing contested the differences [39]), except that the Eu–Yb pair are shown at the right-hand, not left-hand, end. Noticeably, the two-row sequence is fragmented.
Figure 12.7 A dendrogram for the lanthanoids (adapted from Ref. [38]).
Figure 12.8 Lanthanoids arranged according to the cluster analysis of Horovitz and Sârbut (adapted from Ref. [38]).
The clustering of samarium with europium and ytterbium is particularly significant. Together with europium and ytterbium, samarium is the only other lanthanoid to form stable 2+ compounds. In fact, samarium(II) iodide (Kagan’s reagent) is an important mild reducing agent in organic chemistry [40].
The gap between promethium and samarium according to the cluster analysis is interesting in that
