Introduction to the Lanthanides

Lanthanide chemistry started in Scandinavia. In 1794 Johann Gadolin succeeded in obtaining an earth (oxide) from a black mineral subsequently known as gadolinite he called the earth yttria. Soon afterwards, M.H. Klaproth, J.J. Berzelius and W. Hisinger obtained ceria, another earth, from cerite. However, it was not until 1839-1843 that the Swede C.G. Mosander first separated these earths into their component oxides thus ceria was resolved into the oxides of cerium and lanthanum and a mixed oxide didymia (a mixture of the oxides of the metals from Pr through Gd). The original yttria was similarly separated into substances called erbia, terbia, and yttria (though some 40 years later, the first two names were to be reversed ). This kind of confusion was made worse by the fact that the newly discovered means of spectroscopic analysis permitted misidentifications, so that around 70 new elements were erroneously claimed in the course of the century. 

Nor was Mendeleev s revolutionary Periodic Table a help. When he first published his Periodic Table in 1869, he was able to include only lanthanum, cerium, didymium (now known to have been a mixture of Pr and Nd), another mixture in the form of erbia, and yttrium unreliable information about atomic mass made correct positioning of these elements in the table difficult. Some had not yet been isolated as elements. There was no way of predicting how many of these elements there would be until Henry Moseley (1887-1915) analysed the X-ray spectra of elements and gave meaning to the concept of atomic number. He showed that there were 15 elements from lanthanum to lutetium (which had only been identified in 1907). The discovery of radioactive promethium had to wait until after World War 2. 

It was the pronounced similarity of the lanthanides to each other, especially each to its neighbours (a consequence of their general adoption of the 4-3 oxidation state in aqueous solution), that caused their classification and eventual separation to be an extremely difficult undertaking. 

Lanthanide and Actinide Chemistry S. Cotton 2006 John Wiley Sons, Ltd. 

Subsequently it was not until the work of Bohr and of Moseley that it was known precisely how many of these elements there were. Most current versions of the Periodic Table place lanthanum under scandium and yttrium. 

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A. Electron-Phonon Interaction Parameterization Scheme. In observing the fluorescence decay rate from a given J-manifold, it is generally found that the decay rate is independent of both the crystal-field level used to excite the system and the level used to monitor the fluorescence decay. This observation indicates that the crystal-field levels within a manifold attain thermal equilibrium within a time short compared to the fluorescence decay time. To obtain this equilibrium, the electronic states must interact with the host lattice which induces transitions between the various crystal-field levels. The interaction responsible for such transitions is the electron-phonon interaction. This interaction produces phonon-induced electric-dipole transitions, phonon side-band structure, and temperature-dependent line widths and fluorescence decay rates. It is also responsible for non-resonant, or more specifically, phonon-assisted energy transfer between both similar and different ions. Studies of these and other dynamic processes have been the focus of most of the spectroscopic studies of the transition metal and lanthanide ions over the past decade. An introduction to the lanthanide work is given by Hiifner (39). 

Pyrrole ligands can form both Ln-N or-bonds and tjs-n-Ln bonds. complexes with sterically less crowded pyrrole ligands [195]. The introduction of sterically demanding groups in a-position as in 2,5-di-fert-butylpyrrole led to a shielding of the nitrogen and subsequent -coordination to the lanthanide center [196]. Additionally, rj1-coordination to a sodium atom is observed in the obtained ate complex. 

S.A. Cotton (1991) Lanthanides and Actinides, Macmillan, London - A good general introduction to the /-block elements. 

For a general introduction to the field of NMR in magnetically ordered compounds with non-S state lanthanide ions, we refer the reader to Taylor (1971) or McCausland and Mackenzie (1980), where the interplay of the exchange and crystal-field interactions has been analysed. Generally, for cubic systems like RAI2 at least a three-parameter mean-field model is adopted, based on a single-ion Hamiltonian comprising a crystal field of cubic symmetry [CEF parameters W, x Barnes (1979)] and an isotropic molecular field constant (A) ... 

Since we essentially deal with transition-metal compounds in this chapter, we, in this section, present a brief introduction to the main characteristics of rare earth complexes, where the reader can find references to more detailed accounts. The lanthanides show a marked preference for the - -3 oxidation state, and present very similar coordination and organometallic chemistries, with small differences associated only with changes in the atomic size along the series. The interest for the incorporation of lanthanides in supramolecular architectures is associated with Iheir magnetic and light emitting properties. 

The review begins with a selective summary of the history of organome-taUic and reductive rare-earth chemistry as an introduction to bimetallic lanthanide complexes bridged by ligands derived from arenes and to complexes of ferrocene-based diamide ligands, a major subtheme in the chapter. Detailed discussions then follow on complexes derived from fused rings and their reactivity with phosphorus and on complexes derived from biphenyl and stilbene as substrates, with emphasis on ferrocene-based diamide complexes. 

General introductions to the subject of EPR spectroscopy may be found in a wide range of books and reviews. Among those emphasizing the study of transition metal ions, including lanthanides, are Low (1960), Abragam and Bleaney (1970), and Al tshuler and Kozyrev (1974). 

Even a brief introduction to the wide variety of lanthanide minerals exceeds the scope of this chapter. Properties of many are given by Roberts et al. (1974), Palache et al. (1951, vols. 1 and 2). Deer et al. (1962, vols. 1 and 5), Frondel (1958), Fleischer (1966), and Vlasov (1966, vol. 2). Extensive bibliographies have been compiled by Adams and Iberall (1973), Mineev (1969), and Paster (to be published). 

It is well known, that in aqueous solutions the water molecules, which are in the inner coordination sphere of the complex, quench the lanthanide (Ln) luminescence in result of vibrations of the OH-groups (OH-oscillators). The use of D O instead of H O, the freezing of solution as well as the introduction of a second ligand to obtain a mixed-ligand complex leads to either partial or complete elimination of the H O influence. The same effect may be achieved by water molecules replacement from the inner and outer coordination sphere at the addition of organic solvents or when the molecule of Ln complex is introduced into the micelle of the surfactant. 

As mentioned in the introduction, early transition metal complexes are also able to catalyze hydroboration reactions. Reported examples include mainly metallocene complexes of lanthanide, titanium and niobium metals [8, 15, 29]. Unlike the Wilkinson catalysts, these early transition metal catalysts have been reported to give exclusively anti-Markonikov products. The unique feature in giving exclusively anti-Markonikov products has been attributed to the different reaction mechanism associated with these catalysts. The hydroboration reactions catalyzed by these early transition metal complexes are believed to proceed with a o-bond metathesis mechanism (Figure 2). In contrast to the associative and dissociative mechanisms discussed for the Wilkinson catalysts in which HBR2 is oxidatively added to the metal center, the reaction mechanism associated with the early transition metal complexes involves a a-bond metathesis step between the coordinated olefin ligand and the incoming borane (Figure 2). The preference for a o-bond metathesis instead of an oxidative addition can be traced to the difficulty of further oxidation at the metal center because early transition metals have fewer d electrons. 

Relaxivity is the key determinant for the efficacy of lanthanide chelates. The design of improved contrast agents is only possible on the basis of an in-depth understanding of the underlying principles. The third chapter gives an extensive introduction into the theory and the determination of relaxivity data, which will enable the reader to better utilize further optimization efforts. 

Following the introduction to size-dependent nanophenomena presented in the previous sections, we now focus our attention on the luminescence properties of lanthanide ions at additional sites or distorted structure existing in nanophases. Phenomena of prolonged luminescence lifetime, anomalous thermalization, upconversion luminescence, dynamics of long-range interaction with two-level-systems (TLS), and quantum efficiency are to be discussed. 

The introduction of the btsa ligand into lanthanide chemistry by Bradley led to the isolation of the homoleptic compounds Ln(btsa)3 [7], interpreted as the first 3-coordinate lanthanide complexes. This result was indeed spectacular and exploration of the chemical and physical properties of this simple system is continuing [104-106]. 

Although the lanthanides are not usually considered to be transition metals, they are sometimes referred to as inner transition metals because the third transition series begins with La and the next elements are the lanthanides. Accordingly, a brief introduction to their properties and chemistry is included here. 

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