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Chapter 12
Lanthanoids, Group 3, and Their Connections
The short form of the conventional Periodic Table may be convenient and compact, but it results in a marginalization of the lanthanoids (and of the actinoids, covered in Chapter 13). Not only do they appear to be the orphan elements, but in undergraduate chemistry courses, if they are mentioned at all, they are dismissed in the closing lecture(s) of the course as being boring and of little interest. Yet this is not the case. In addition to some trends, there are also interesting and curious exceptions. Lanthanoid chemistry is most definitely worthy of study.
In Chapter 5, the rare earth metals were defined as the lanthanoids (see in the following) plus the two earlier Group 3 elements of scandium and yttrium. Cotton has referred to these two elements as “The Misfits” of the Periodic Table [1] and these two elements will be discussed first.
Yttrium and Scandium
In the debate on the 6th Period and 7th Period members of Group 3 (see Chapter 4), the 4th Period and 5th Period members of the Group are often overlooked. In Chapter 8, scandium and yttrium were rejected from membership of the transition metals. They have found a home in this book by joining the lanthanoids as part of the rare earth metal grouping.
Yttrium
The yttrium 3+ ion resembles the lanthanoid 3+ ions so closely that it is best regarded as a later lanthanoid. For example, yttrium’s standard reduction potential is −2.37 V compared with −2.37 V for lanthanum and −2.30 V for lutetium. Also, the yttrium 3+ ion has an ionic radius of 90.0 pm compared with 90.1 pm for holmium.
In the context of its chemistry, the yttrium(III) halides are isostructural with all the lanthanoid(III) halides from dysprosium to lutetium. The yttrium(III) ion exists as an octahydrate is aqueous solution, [Y(OH2)8]3+, as do many of the lanthanoids. The major interest in ytterbium has been for yttrium oxosulfide, Y2O2S, doped with later lanthanoid(III) ions as long-lasting phosphors [2].
Though yttrium seems “more comfortable” with the latter lanthanoids, curiously, yttrium in minerals is usually associated with the earlier lanthanoids. Two examples of these are bastnäsite, (Ce,La,Y)CO3F, and gadolinite (which contains no gadolinium), (Ce,La,Nd,Y)2FeBe2Si2O10.
Though yttrium does seem to be predominantly lanthanoid-like, there seems to be a significant resemblance in chemistry to its (n + 10) Group 13, analog, indium (see Chapter 9). Interestingly, research on inorganic polymers has also found some similarities of yttrium and indium [3].
Scandium
While yttrium behaves unambiguously like a lanthanoid, the much smaller scandium(III) ion (74.5 pm) has resemblances to — and differences from — both transition metals and lanthanoids. In fact, Cotton’s term of “misfit” does indeed apply to this element. For example, like the transition metal ions from titanium(III) to cobalt(III), scandium(III) forms a three-coordinate silylamide, Sc[N(Si(CH3)3)2]3. However, while the transition metal complexes are planar, that of scandium is pyramidal. Despite the difference in ion size, scandium can be found with yttrium in such ores as thortveitite, (Sc,Y)2Si2O7.
In Chapter 9, the several similarities of scandium to aluminum were discussed in the context of the (n) and (n + 10) relationship. The major use for scandium is in specialty alloys of aluminum, the Al3Sc micrograins imparting additional strength to the aluminum metal [4]. In fact, in some respects, scandium is closer in chemistry to aluminum than to any other element.
The 4f Elements
When the first rare earth elements were discovered, the question arose as to where they could be fitted in the eight-group Periodic Table. At the time, it was believed that the eight-group framework was the key “set in stone” feature of the Periodic Table: the elements themselves were the problem. It was not until 1882 that Bailey concluded that the known rare earth elements were not members of any of the eight Groups [5]. Then Bassett, in 1892, proposed that the lanthanoid elements (as we now call them) form their own series (and that the known actinoids formed a matching series). The key to the understanding of the lanthanoids came in 1921, when Bury postulated that they corresponded to the filling of the 4f orbitals.
Progressing to the present, what of their chemistry? Pimentel and Sprately summed up the opinion of most chemists to the chemistry of the 4f elements [6]:
Lanthanum has only one important oxidation state in aqueous solution, the +3 state. With few exceptions, this tells the whole boring story about the other lanthanides.
Yet the predominance of the +3 state is one of the very interesting things about the 4f elements. Nowhere else in the Periodic Table is it possible to study a sequential series of elements all in the same oxidation state. And there are many other interesting aspects to their chemistry as will be covered in the following.
Books specifically on the 4f elements (or the “rare earth” elements, which also encompass Group 3 elements — see Chapter 4) are very rare. One volume was part of a series on each segment of the Periodic Table [7]. An undergraduate textbook claiming to be on the d-block and f-block elements contained very little on the f-block, instead being almost entirely on the d-block [8]. Though the text by Cotton (mentioned earlier) [1] was comprehensive, it did not look beyond the confines of the f-block for relationships, nor did the
