Divalent rare earth metals

Boff and Novae [6] found a divalent rare earth metal complex, (C5Me5)2Sm, to be a good catalyst for the polymerization of MMA. The initiation started with... 

ALLOYS OF THE ALKALINE EARTH METALS AND OF THE DIVALENT RARE EARTH METALS... 

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. 

In spite of the fact that in many cases the larger alkaline earth metals can be exchanged for divalent rare earth metals, the latter ones form different varieties of three-dimensional or layered triply bonded disilicides with either the a-XhSi2 or the AIB2 structure. These two structure types are clearly dominated by a trigonal prismatic arrangement of the cations and contain silicon in exclusively trigonal planar coordination (Fig. le and If). 

RXH. The hydride halides RXH of the divalent rare earth metals have been known for a long time. All of them, EuXH, YbXH with X = Cl, Br, 1, and SmBrH (Beck and Limmer 1982) crystallize in the PbFCl-type structure, which is also adopted by the hydride halides of the alkaline earth metals MXH (Ehrlich et al. 1956), by the mixed halides RXX of divalent lanthanides, and many oxyhalides ROX of the trivalent metals. The colorless compounds RXH of R = Sm, Eu, Yb therefore have to be addressed as normal salts. The hydrogen content of these compounds is strictly stoichiometric. 

Mitzi, D.B. Liang, K.N. Preparation and properties of (C4H9NH3)2Eul4 A luminescent organic-inorganic perovskite with a divalent rare-earth metal halide framework. Chem. Mater. 1997, 9, 2990. 

Besides isolated divalent rare earth metal complexes, systems that behave as M(ll) are usually included with low-valent organometallic rare earth... 

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). 

The 3rd group metals a summary of their atomic and physical properties 5.5.5.1 The rare earth metals. A summary of the main atomic and physical properties of the rare earth metals has been collected in Tables 5.11-5.13. To complete the information and the presentation of the entire series of lanthanides the data relevant to Eu and Yb have been included in these tables. However, the same data are reported also in Table 5.7 in comparison with those of the other typical divalent metals (the alkaline earth metals). As for the properties of liquid rare earth metals and alloys see Van Zytveld (1989). 

The rare earth metals. The rare earth metals are extremely reactive elements especially with respect to the normal atmospheric gases. The light trivalent lanthanides oxidize with air at room temperature they should be stored (and handled) in vacuum or under He or Ar. Divalent Eu oxidizes much more readily than any of... 

Fig. 9.5. The variation exchange splittings, AE, in the 4s and 5s regions of rare earth metals and trifluorides through the lanthanide series. The lines drawn are the calculated exchange splittings, showing a good agreement with A E5S and a large overestimation for A 4. Eu and Yb show deviations indicating their divalent nature [50].

For mono-metallofullerenes with rare earth metals, three electrons are donated from the encaged metal to the fullerene cage, such as Y and most lanthanides (La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er, andLu) [2, 108, 109]. However, Sc, Sm [110], Eu [111], Tm [112], and Yb [113] are found to donate only two electrons and prefer to take the valence of +2. Consequently, the EMEs containing rare earth metals are classifiable into two categories according to the electronic state of the encaged metal(s) trivalent EMFs and divalent EMFs. Eor example, La Cs2 should be expressed as La + Cg2. whereas Yb Cs2 has the formula of Yb + CgJ. These two types of EME differ markedly from each other in their structures and properties, (i) When three electrons are transferred to the carbon cage, the resultant -type EMFs... 

A second commonly invoked kinetic model for ETU processes is the so-called Dimer Model [22,23], in which pairs are treated as distinct isolated entities. Such scenarios are often encountered when trivalent rare-earth metal ions are substituted into divalent host lattices of the CsNiClj type. In these hosts, only small concentrations of M + ions can typically be incorporated, and it has been shown that more than 90% of all ions are introduced as (M + - vacancy - M +) pairs to satisfy charge compensation requirements [24, 25]. The ions are thus introduced as isolated pairs. In this model, three excitation populations are con-... 

A new breakthrough in the non-classical divalent rare-earth chemistry occurred in 1997, when Bochkarev and Evans in a milestone paper reported that a light-sensitive molecular complex of Tml2, identified as a solvate of composition [Tml2(DME)3] (Figure 1) (DME = 1,2-dimethox-yethane), could be made in relatively mild conditions by the direct reaction of thulium metal with iodine in refluxing DME (Scheme 1). 

Tml2, Dyh and Ndh have also been used in an acetonitrile/amine coupling reaction, which produced amidines of general formula MeC (=NH)NR R R R2 = H, Me H, iPr H, fBu Et2). The reaction is sub-stoichiometric in rare-earth diiodide but not really catalytic since part of the produced amidine remained tightly held aroimd the rare-earth metal it could be liberated by heating a trivalent intermediate formulated as Rl2(amidine)4(amidinate) (R = Nd, Dy, Tm) imder vacuum, and the residue could be recycled to produce more amidine. This reaction is not specific of the divalent iodides since many rare-earth triiodides were also effective. In the case of dysprosium and diethylamine, an intermediate trivalent amidine complex has been isolated and structurally characterised in the form of the zwitterionic [Dy MeC(=NH)NEt2 4][(I)3] (Bochkarev et al., 2007) (Figure 9). 

All the rare earth metals, in the form of powders mixed with alkali chloride, may be prepared by this method. In preparing Sm, Eu or Yb metals (these elements form divalent compounds), a temperature of 250°C must not be exceeded, since at higher temperatures, the direction of the reaction is reversed and SmCls, EuCls and YbCls are formed. 

Cerium oxide, ceria, has a fluorite structure and shows oxide anion conducting behavior differ from other rare earth oxides. However, the O ionic conductivity of pure ceria is low because of a lack of oxide anion vacancies. For ion conduction, especially for anion, it is important to have such an enough vacancy in the crystal lattice for ion conduction. Therefore, the substitution of tetravalent Ce" by a lower valent cation is applied in order to introduce the anion vacancies. For the dopant cation, divalent alkaline earth metal ions and some rare earth ions which stably hold trivalent state are usually selected. Figure 9-28 shows the dopant ionic radius dependencies of the oxide ionic conductivity for the doped ceria at 800°C. In the case of rare earth doped Ce02, the highest O ion conductivity was obtained for... 

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