Ytterbium divalent

The solid solubility behavior of the divalent barides (europium and ytterbium = M) are interesting in that 5 to 20at.% of M is soluble in R, but little or no solubility of R in M is found. Since the alkaline earth metals (M) appear to be insoluble in R (and also R in M), it appears that the tendency of these two bivalent metals (Eu and Yb) to form a trivalent state in the trivalent R matrix accounts for this extensive solubility. But in the europium or ytterbium divalent matrix the trivalent rare earth metals are essentially insoluble because of their smaller size and different electronic (trivalent) nature. 

UV-visible absorption spectroscopy of Ln " ions has been widely studied. A lot of data are also available in the case of Yb + and Sm " in a host lattice (Dyke et al., 1972 Fong, 1967 Johnson et al., 1969 McClure et al., 1968 Wang et al., 1973). Some studies deal with the absorption spectra of aqueous Sm + and Yb " " (Butement, 1948 Farragi et al., 1972 Ganopol skii et al., 1966 Johnson et al., 1968). Only a few results are available in the case of molecular samarium and ytterbium divalent compounds in an organic medium (see table 4). 

Although rare-earth ions are mosdy trivalent, lanthanides can exist in the divalent or tetravalent state when the electronic configuration is close to the stable empty, half-fUed, or completely fiUed sheUs. Thus samarium, europium, thuUum, and ytterbium can exist as divalent cations in certain environments. On the other hand, tetravalent cerium, praseodymium, and terbium are found, even as oxides where trivalent and tetravalent states often coexist. The stabili2ation of the different valence states for particular rare earths is sometimes used for separation from the other trivalent lanthanides. The chemicals properties of the di- and tetravalent ions are significantly different. 

While ytterbium(II) benzamidinate complexes have been known for many years/ the synthesis of the first divalent samarium bis(amidinate) required the use of a sterically hindered amidinate ligand, [HC(NDipp)2l (Dipp = C6H3Pr2-2,6)/ As illustrated in Scheme 54, the dark green compound Sm(DippForm)2(THF)2 (DippForm = [HC(NDipp)2] ) can be prepared by three different synthetic routes. Structural data indicated that hexacoordinated... 

Figure 4.17. The binary phase diagrams of the magnesium alloy systems with the divalent metals ytterbium and calcium (Ca is a typical alkaline earth metal and Yb one of the divalent lanthanides). Notice, for this pair of metals, the close similarity of their alloy systems with Mg. The compounds YbMg2 and CaMg2 are isostructural, hexagonal hP12-MgZn2 type.

On the other hand it may be noticed that some aspects of the chemistry and alloying behaviour of Ca, Sr and Ba could be conveniently compared with those of the divalent rare earth metals europium and ytterbium. 

Within the lanthanides the first ones from La to Eu are the so-called light lanthanides, the other are the heavy ones. Together with the heavy lanthanides it may be useful to consider also yttrium the atomic dimensions of this element and some general characteristics of its alloying behaviour are indeed very similar to those of typical heavy lanthanides, such as Dy or Ho. An important subdivision within the lanthanides, or more generally within the rare earth metals, is that between the divalent ones (europium and ytterbium which have been described together with other divalent metals in 5.4) and the trivalent ones (all the others, scandium and yttrium included). 

Europium and ytterbium di-valence. The oxidation state II for Eu and Yb has already been considered when discussing the properties of a number of divalent metals (Ca, Sr, Ba in 5.4). This topic was put forward again here in order to give a more complete presentation of the lanthanide properties. The sum of the first three ionization enthalpies is relatively small the lanthanide metals are highly electropositive elements. They generally and easily form in solid oxides, complexes, etc., Ln+3 ions. Different ions may be formed by a few lanthanides such as Ce+4, Sm+2, Eu+2, Yb+2. According to Cotton and Wilkinson (1988) the existence of different oxidation states should be interpreted by considering the ionization... 

Figure 5.8. Lanthanide Ln203 oxides (cubic cI80-Mn2O3 type, on the left side) and Pb alloys (LnPb3, cubic cP4-type, on the right). The trends of the lattice parameter and of the heat of formation are shown (see the text and notice the characteristic behaviour of Eu and Yb). A schematic representation of the energy difference between the divalent and trivalent states of an ytterbium compound is shown. Apromff represents the promotion energy from di- to trivalent Yb metal, A,//11, and Ar/Ynl are the formation enthalpies of a compound in the two cases in which there is no valence change on passing from the metal to the compound the same valence state (II or III) is maintained.

The rare earth (RE) ions most commonly used for applications as phosphors, lasers, and amplifiers are the so-called lanthanide ions. Lanthanide ions are formed by ionization of a nnmber of atoms located in periodic table after lanthanum from the cerium atom (atomic number 58), which has an onter electronic configuration 5s 5p 5d 4f 6s, to the ytterbium atom (atomic number 70), with an outer electronic configuration 5s 5p 4f " 6s. These atoms are nsnally incorporated in crystals as divalent or trivalent cations. In trivalent ions 5d, 6s, and some 4f electrons are removed and so (RE) + ions deal with transitions between electronic energy sublevels of the 4f" electroiuc configuration. Divalent lanthanide ions contain one more f electron (for instance, the Eu + ion has the same electronic configuration as the Gd + ion, the next element in the periodic table) but, at variance with trivalent ions, they tand use to show f d interconfigurational optical transitions. This aspect leads to quite different spectroscopic properties between divalent and trivalent ions, and so we will discuss them separately. 

The reduction to the divalent state involves samarium, europium, and ytterbium. In 1906 C. Matignon and E. Gazes obtained samarium(II) chloride by reducing the trichloride with hydrogen. In 1911, G. Urbain and F. Bourion prepared europium(II) chloride by a comparable reduction involving gydrogen, and in 1929 ytterbium(II) chloride was similarly obtained by W. Klemm and W. Schuth. 

H.B. Kagan and J.L. Namy, Preparation of divalent ytterbium and samarium derivatives and their use in organic chemistry 525... 

In 1956 it was found that europium and ytterbium dissolve in liquid ammonia with the characteristic deep blue color known for the alkali and alkaline earth metals [36-40]. This behavior arises from the low density and high volatility of those metals compared to the other lanthanide elements [41]. Samarium, which normally also occurs in the divalent oxidation state, does not dissolve under... 

W. J. Evans, M. A. Johnston, M. A. Greci, and J. W. Ziller, Synthesis, Structure, and Reactivity of Unsolvated Triple-Decked Bent Metallocenes of Divalent Europium and Ytterbium, Organometallics 18, 1460-1464 (1999). 

P. Girard, J. L. Namy, and H. B. Kagan, Divalent lanthanide derivatives in organic synthesis. 1. Mild preparation of samarium iodide and ytterbium iodide and their use as reducing or coupling agents,. /. Am. Chem. Soc., 102 (1980) 2693-2698. 

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