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Encyclopedia of Physical Science and Technology EN014A-653 July 28, 2001 20:55

16 Rare Earth Elements and Materials

FIGURE 11 Molecular structures of three commonly used ligands for rare earth organometallic compounds.

(CO molecules as ligands) are not stable due to the lack of IV. TECHNOLOGICAL APPLICATIONS
any significant back-bonding. The metal–ligand bonding
is primarily ionic in character because of the contracted Rare earth materials represent a growing market and group
nature of the 4 f orbitals. The highly carbanionic character of technologies. These materials have expanding applica-
of organic ligand and oxophilicity of rare earth ions render tions in markets as diverse as rechargeable batteries, ad-
these substances extremely air and moisture sensitive. vancedceramics,permanentmagnets,opticaldatastorage,
Although the organometallic chemistry of the rare earths laser, fiber optics, glass, phosphors, and superconductors.
is not as extensive as that of their d-block counterparts, the Clearly it will be impossible to discuss each of these fully,
research is currently receiving a lot of attention, especially so we will restrict our list to the major categories and treat
in the applications of these substances as highly efficient only a few in modest detail.
catalysts in organic synthesis and polymer chemistry.
The organometallic chemistry of the rare earths is pre-
dominantly, though not exclusively, that of cyclopen- A. Phosphors
tadienyl and substituted cyclopentadienyl compounds
A phosphor is a solid luminescent material that converts
(Fig. 11). These complexes were the first organometal-
certain types of energy into light (electromagnetic radia-
lic rare earths to be synthesized and comprise three se-
tion). The intense emissions and almost monochromatic
ries: RE(C 5 H 5 ) 3 , RE(C 5 H 5 ) 2 X, and RE(C 5 H 5 )X 2 . They
tonesobtainedbydilutingtherareearth-basedactivatorsin
are prepared by the reaction of the lanthanide chloride
theappropriatehostnetworksrenderrareearthcompounds
with the stoichiometric amount of NaC 5 H 5 :
very attractive for phosphor application, especially to meet
RECl 3 + n C 5 H 5 Na ⇒ RECl 3−n (C 5 H 5 ) + nNaCl. the very specific criteria for use that traditional band emis-
n
sion phosphors could not satisfy. A great variety of emis-
The properties of cyclopentadienyl rare earth compounds sions can be obtained, depending on the type of activator
are influenced markedly by the relationship between the brought into play and the respective positions of the ex-
size of the rare earth atom and the steric demand of the cited or fundamental energy levels. Rare earth phosphors
cyclopentadienyl group. havenowbeenextensivelyusedincolortelevisionscreens,
Rare earth organometallics have also been prepared
with cyclooctatetraenyl (Fig. 11) and arene ligands, so
have the σ bonded alkyls and aryl compounds. In the lat-
ter case, bulky and/or chelating ligands are typically used
in order to achieve steric saturation around the rare earth
center and hence the stability of the compounds.
Divalent rare earth organometallics for Eu, Yb, and Sm
have also been prepared. The most important compounds
are those of the pentamethylcyclopentadienyl ligands.
The products have bent structures as have, surpris-
ingly, the solvent free (C 5 Me 5 ) 2 RE (RE = Sm, Eu).
Remarkably, (C 5 Me 5 ) 2 Sm reversibly complexes with
dinitrogen, forming a dimeric compound with side-on FIGURE 12 Molecular structure of a dinitrogen complex of
bridging nitrogen (Fig. 12). samarium(II) pentamethylcyclopentadienylide, (C 5 Me 5 ) 2 Sm.