Yet in the research world, lanthanoids have been a burgeoning field. The metals and their alloys have become indispensable as magnetic materials [11]. In the 1990s, the primary interest in their compounds was as reagents in organic synthesis [12, 13]. Now the focus has become on the luminescent properties of the lanthanoid ions and their applications [14–16]. There is also an interest in a group of extremophile aerobic methanotrophs that require one of the early lanthanoids (lanthanum, cerium, praseodymium, or neodymium) for their metabolic pathways [17].
The Lanthanoids
To begin, as is recognized by the International Union of Pure and Applied Chemistry (IUPAC), the correct term is “lanthanoids” [18]. The ending “-ide” is accepted throughout chemistry nomenclature as referring to a negative ion, as in “oxide” or “sulfide.” The commonly accepted definition of a lanthanoid is therefore:
Figure 12.1 The lanthanoid elements as defined in this chapter.
A lanthanoid is any of the series of 15 consecutive chemical elements in the Periodic Table from lanthanum to lutetium (Figure 12.1).
Properties of the Elements
There is the inference that, for a particular property of the lanthanoid elements, a linear or smooth curve plot results. This is not necessarily true. Cater showed that for several parameters of high-temperature lanthanoid chemistry, the plots are much more uneven [19]. This discovery was revisited by Johnson, who came to the following conclusion [20]:
… the lanthanide elements behave similarly in reactions in which the 4f electrons are conserved, and very differently in reactions in which the number of 4f electrons changes.
Laing showed there were significant deviations from linearity for lanthanoid melting points. Though there is a general trend of increasing melting points of the lanthanoids with increasing atomic number, there are two exceptions to the rule: europium and ytterbium, which both have melting points well below that of the trend. He ascribed these anomalies to much weaker metallic bonds for these two elements [21].
In addition, Laing noted that the densities of the lanthanoid metals followed an even more linear relationship (see Figure 12.2), though again, with the exception of europium and ytterbium [21]. He accounted for these two deviations in terms of the electron-sea model of metallic bonding. For all other lanthanoids, the intermetallic forces involved 3+ ions and the intervening three “roaming” valence electrons. Laing argued that as europium and ytterbium favored the 2+ state (see in the following), then the intermetallic forces between the (theoretical) 2+ ions and two “roaming” electrons would be significantly less.
Figure 12.2 Densities of the elements from atomic number 56 to 72 (adapted from Ref. [20]).
However, though the electron-sea model can provide a very simplistic idea of metal behavior, it is incapable of being given any quantitative or even semiquantitative validity. Without going into the sophistication of band theory, the alternative is the soft-sphere model. Lang (not to be confused with Laing) has applied the soft-sphere model to the lanthanoid metals and showed that it is a good fit [22]. Nevertheless, it is more of a justification than an explanation, in that the calculation requires knowledge and use of the crystal packing factor. It does not explain why europium uniquely has the body-centered cubic packing rather than the more compact (and therefore denser) packing arrangements of the other lanthanoids.
Ion Charges of the Lanthanoids
The 3+ state predominates for all of the lanthanoids [23]. It is the 3+ oxidation state consistency that gives such a useful comparison across the series.
The Lanthanoid Contraction
Of importance is the 3+ ionic radii of the lanthanoids. As can be seen from Figure 12.3, the ionic radius decreases, almost linearly, from lanthanum to lutetium. This decrease is known as the lanthanoid contraction (or more commonly as the “lanthanide contraction”) [24]. The effect was first recognized and named by the Norwegian geochemist, Victor Goldschmidt. The contraction, defined in the following, is an important aspect of lanthanoid and, even postlanthanoid chemistry [25].
The lanthanoid contraction is the greater-than-expected decrease in ionic radii of the elements from lanthanum to lutetium, which results in smaller than otherwise expected ionic radii for the subsequent elements commencing with hafnium.
The lanthanoid contraction can be explained as follows [26]. It is the inner, filled, 5s25p6 electron “layer” that defines the ionic radius. The 4f electrons contribute little to the shielding. Thus, as the nuclear charge increases, there is a contraction of the 5s25p6 orbitals causing a radius reduction of the ions.
Figure 12.3 The lanthanoid contraction for the 3+ ions.
It is a common assumption that, having given it a special name, the lanthanoid contraction is greater than other contractions across periods. This is not true, as Lloyd has pointed out [27]. In fact, the decrease in radius from lanthanum(3+) to lutetium(3+) of 117 pm to 100 pm is less than that from calcium(2+) to zinc(2+) of 114 pm to 88 pm. From one ion to its neighbor, the average individual lanthanoid contraction is also less than that from scandium(3+) to gallium(3+) of 89 pm to 76 pm.
Post-Lanthanoid Effect of the Atom and Ion Contraction
As described in Chapter 8, a crucial consequence of the 14-element contraction is that the 6th Period transition metal series are of almost the same atom and ionic radius as their 5th Period analogue. As an example, the pair of zirconium and hafnium can be considered. Comparing these two elements, the number of protons (and number of electrons) has increased from 40 to 78. Yet the atomic radius decreases from 159 pm to 156 pm. Similarly, there is a small decrease, not an increase, in ionic radii from 86 pm (Zr4+) to 85 pm (Hf4+).
Had the lanthanoids not intervened, it is possible to make a rough estimate of the Hf4+ ion radius. This can be done by comparing the ionic radius of the 3+ ion preceding the lanthanoids, lanthanum (117 pm) with the element above it, yttrium (104 pm). Using approximately the same difference, without
