Rare-earth central metals

The first dye used in an electrophosphorescent LED was the terbium-complex Tb(acetyhlacetonate)3 (Tb(acac)3, see Fig. 11.2) [21], LEDs based on complexes with rare-earth central metals such as terbium or europium are very interesting for display applications, because they emit light with a very small spectral line width. These sharp emission lines are due to f-f transitions located on the central metal ion. Disadvantages of these complexes are, however, that color tuning via the chemical modification of the ligand is not possible and that the radiative lifetime of phosphorescence is rather long. 

Polymerization of cycloalkenes through the double bond without ring opening occurs when initiators other than the ROMP initiators are used, including initiators based on various early transition (Ti, Zr, Hf, V), late transition (Pd, Ni), central transition (Cr, Co), and rare-earth (Ln) metals [Janiak and Lassahn, 2001]. This mode of polymerization is discussed in Secs. 8-lf and 8-6a. 

As many studies of gas-solid surface reactions involving rare earth inter-metallics are in some sense or other related to hydride formation, the interaction with hydrogen shall be of central interest in this section. The interaction of hydrogen with the intermetallic compounds under consideration can be represented schematically by the following sequence of processes ... 

Aspinall and coworkers reported detailed structural studies of rare earth-alkali metal heterobimetallic complexes [131]. Whereas the crystal structures provided by Shibasaki and coworkers included one molecule of water coordinating to the central metal, Aspinall and coworkers succeeded in preparing anhydrous crystals of M3[Ln(binol)3] (LnMB). Li3[Ln(binol)3] complexes were obtained from Ln(N(SiMe3)2)3 and Li(Hbinol) in THE or Et20. The resulting HN(SiMe3)2 was removed under reduced pressure, and the crystals were obtained from... 

Electrotransport In this process too a super-clean atmosphere is necessary. In electrotransport (a kind of solid-state electrolysis) a large dc current (typically 200 A cm-2) is passed through a rod of the metal at a temperature 100-200°C below its melting point. In the rare earth metals the interstitial impurities slowly move towards the anode, while several metallic impurities move towards the cathode. In this case too, as in zone melting, the purest portion of the bar is its central part. 

Intramolecular transfers between two ehromphores separated by insulating groups can lead to absorption by one chromophore and emission from the other. Complexes of rare earths specially of Eu+3, Sm+3. Gd+3 and Dy+3 emit line spectrum characteristic of the central metal ion when absorption takes place in the ligand moeity. 

Salicylates.—In the course of his intramolecular energy transfer studies (p. 130) on europium chelates Weissman 630 prepared the 3-nitro and 5-nitro salicylate complexes. He was one of the first to demonstrate the phenomena of intramolecular energy transfer (IMET) from the coordinated ligands to the central rare earth ions giving characteristic fluorescence of the metal ions. However, no analytical data on these compounds are available. 

The chemical composition of rare earth complexes cannot by itself reveal the coordination number of the central metal ion. There are many complexes containing hydrated water molecules and coordinated water molecules. The nitrilotriacetic acid (NTA) complexes of Pr and Dy have the formulae PrNTA 3H2O and DyNTA 4H2O, respectively. The praseodymium complex is a nine-coordinate system with one molecule of water in the hydrated form [12] and the dysprosium complex is eight-coordinate with two molecules of water of hydration [13]. These structures cannot be predicted from the composition of the complexes. The complex Nd(N03>3 4DMSO is ten-coordinate [14] since the nitrate... 

In several rare earth ions, Sc3+, Y3+, La3+, Ce4+ and Lu3+, the core electronic structure has filled shells and as a result no absorption spectra at >200 nm is expected since high energies are required to promote an electron from filled shells. Broad absorption that increases exponentially in the UV region is observed in aquo ions of Eu3+, Yb3+ and Ce4+. Intense bands are observed in complexes like CeCl - and CeBrjj and these bands are thought to be due to electron transfer from the ligand molecular orbital to the central metal ion. 

The effect on the central nervous system of the rare-earth metals following inhalation may preclude welding operations with these materials to any large extent. Cerium is stated to produce polycythemia but is useless in the treatment of anemia owing to its toxic effects. The salts of cerium increase the blood coagulation rate. See also RARE... 

---