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Rare Earth Elements and Materials 15

2. Solution Chemistry of the Rare Earth Ions
Coordination chemistry of rare earth elements is an ac-
tive research theme because of the importance of gadolin-
ium complexes as contrast agents in magnetic reso-
nance imaging, and luminescent europium and terbium
complexes as probes in biochemistry. The extremely
facile ligand exchange at f element centers makes the
isolation of their coordination complexes difficult, par-
FIGURE 9 A view of the atomic arrangement (crystal structure) in
Gd 2 Cl 3 , Y 2 Cl 3 , etc. The Gd atoms are in the shape of octahedra ticularly from aqueous solutions. A successful method of
that share two edges forming an infinite chain shown in heavy overcoming this is to increase the thermodynamic stability
outline. The Cl atoms are attached to the Gd atoms making up
of the complexes by exploitation of the chelate and macro-
the chain. [From Corbett, J. D. (1981). Acct. Chem. Res. 14, 243.]
cyclic effects. Typical ligands used in such a strategy in-
clude β-diketonates, EDTA, and macrocyclic ligands such
as crown ethers and cryptands (Fig. 10). Although the
cannot apply the same concepts that work with oxides to
solution chemistry of the rare earth ions is predominantly
the chalcogenide family.
that of the trivalent oxidation state, the chemistry of lan-
The compounds of composition RES are perhaps the
thanideionsofotheroxidationstateshasalsobeenstudied,
most interesting. These form for all rare earths, yet only 4+ 2+ 2+ 2+
including those of Ce ,Sm ,Eu , and Yb .
for Sm, Eu, and Yb is the rare earth valence 2+. In all
other cases, physical measurements point to the 3+ va-
lence and as only two of the valence electrons go to make C. Organometallics
2−
S the extra electron goes into the conduction electron
The term “organometallics” defines compounds in which
sea making these materials good electrical conductors.
the rare earth atom is bound to carbon through single σ-
The rare earth carbides, nitrides, and borides have also
bonds or through multiple π-bonding. The organometallic
been prepared, but these substances are much less well
chemistry of the rare earths is arguably the most significant
developed and will not be discussed here.
development in rare earth chemistry in the last 2 decades.
As compared with other metals, the distinct features
B. Coordination Compounds of rare earths include not only their highly positive
characteristics, like that of alkali metals, but also a wide
1. General Features of Coordination
coordination sphere, like that of d-block elements. The
Chemistry of Rare Earths
properties and the structure of rare earth organometallics
The rare earth ions typically exist in the trivalent oxidation reflect the peculiarity of the electron arrangement of these
state. The cations show similar properties, with gradual metals. For example, rare earths cannot act as π-electron
changes occurring across the lanthanide series, a size ef- donors and, as a result, rare earth carbonyl compounds
fect from the lanthanide contraction. Because of the large
size of the cations, a coordination number greater than 6
(usually 8 to 9) in their compounds is often adopted. In
addition, because the f electrons are well shielded from
their environment, the effects imposed by the ligand(s)
are insignificant, leading to the primarily ionic bonding
in the coordination compounds. The lack of covalency in
RE–ligand interaction implies that there are no orbital re-
quirements for ligands around the metal center; the coordi-
nation polyhedra are determined by the steric requirement
of the ligands and are therefore often ill defined. The ionic
bonding character also causes the rapid ligand exchange,
and as a result, stable complexes of the rare earth ions
can only be obtained with chelating ligands. RE 3+ cations
display typical hard Lewis acid properties that prefer co-
ordination with O and F–donor ligands. One indication is
that the products isolated from an aqueous solution are FIGURE 10 Molecular structures of three commonly used chelat-
often found containing aqua ligand(s). ing ligands for rare earth coordination compounds.