Similarity scandium-based

Lanthanide elements (referred to as Ln) have atomic numbers that range from 57 to 71. They are lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu). With the inclusion of scandium (Sc) and yttrium (Y), which are in the same subgroup, this total of 17 elements are referred to as the rare earth elements (RE). They are similar in some aspects but very different in many others. Based on the electronic configuration of the rare earth elements, in this chapter we will discuss the lanthanide contraction phenomenon and the consequential effects on the chemical and physical properties of these elements. The coordination chemistry of lanthanide complexes containing small inorganic ligands is also briefly introduced here [1-5]. 

Rare earth elements have similar configurations in the two outermost shells. They exhibit typical metallic properties in chemical reactions. They tend to lose three electrons and exhibit a 3+ valence state. From the Periodic Table of the elements, rare earth elements are classed as less reactive than alkali metals and alkaline earth metals but more reactive than other metals. They should be stored in an inert liquid otherwise they will be oxidized and lose their metal luster. The metal reactivity increases gradually from scandium to lanthanum and decreases gradually from lanthanum to lutetium. That is to say, lanthanum is the most reactive metal of the 17 rare earth elements. Rare earth metals can react with water and release hydrogen. They react more vigorously with acids but do not react with bases. 

A few examples are known of crystal structures based on more complex nets. Borocarbides MB2C2 are formed by scandium and the rare-earth metals, and they consist of layers of composition B2C2 interleaved with metal atoms. In the compounds of the 4f metals the layer is the 4 8 layer, the pattern of atoms being similar to that of Fig. 3.27(b), the open and shaded circles now representing B and C atoms. The scandium compound is of special interest as the only example at present known of the layer consisting of equal numbers of 5-gons and 7-gons. 

The base-free dimethyl Sc complex 121 was a highly active catalyst precursor for ethylene polymerization under B(C6F5)3, trityl borate, or methylaluminoxane (MAO)-type activation. The catalytic activity of 121 was similar to those observed of Group 4 metallocene complexes [81]. Generally, cationic scandium complexes are believed to be the active species. Activation of the catalyst was studied by reacting 120 and 121 with various equivalences of B(C6F5)3. The monomeric bulky rBu-substituted dimethyl complex 121 reacted with 1 equiv of B(C6F5)3... 

A similar approach is used in CO2 gas sensors based on electrolyte chains of YSZ and cation conductors. Specifically, YSZ has been used with magnesium- [223], aluminum- [96, 105, 224, 225], or scandium- [225-227] conducting electrolytes and Li2CO3-containing electrodes. The sensitivity to CO2 is attributed to the dissolution of lithium in the electrolyte rather than to the formation of a new carbonate phase. 

As for the chiral ytterbium and scandium catalysts, the following structures were postulated. The unique structure shown in scheme 13 was indicated by 13C NMR and IR spectra. The most characteristic point of the catalysts was the existence of hydrogen bonds between the phenolic hydrogens of (R)-binaphthol and the nitrogens of the tertiary amines. The 13 C NMR spectra indicated these interactions, and the existence of the hydrogen bonds was confirmed by the IR spectra (Fritsch and Zundel 1981). The coordination form of these catalysts may be similar to that of the lanthanide(III)-water or -alcohol complex (for a review see Hart 1987). It is noted that the structure is quite different from those of conventional chiral Lewis acids based on aluminum (Maruoka and Yamamoto 1989, Bao et al. 1993), boron (Hattori and Yamamoto 1992), or titanium... 

Scandium, yttrium, and the lanthanides represent 17% of all of the elements that can be obtained in coherent form. In view of their similar chemical behavior separation of these elements proved to be formidable (Szabadvary, 1988) but all of the metals have now been obtained in relatively high purity and, except for promethium, their thermodynamic properties have been measured with varying degrees of quality. Based on the interpolation of the properties of neighboring elements, it has also been possible to estimate the thermodynamic properties of promethium in order to complete this review. Because of the radioactive nature of this element and the fact that the most stable isotope has half-life of only 17.7 years, it is unlikely that further measurements will be carried out beyond the very basic properties which have already been determined. 

It was not until 1859 that Bunsen first applied the spectrograph to analytical chemistry determinations, and this development proved useful in the case of the rare earths. The nature of the spectra of the various rare earths was not understood until well into the Twentieth century, so the analytical methods were empirical and not always dependable. The uncertainty was due to the fact that the transition elements also separated along with the rare earths in the fractionation process, and tended to complicate the various spectra obtained. As a result of these complications, the discovery of over 70 new rare earths was reported in the literature. Many of the new elements were based on spectra differences in the fractions obtained, and no one knew how many rare earths should exist. It was not until 1869 that Mendeleyev published his first periodic chart. Incidentally, in doing so, he had to leave a blank where scandium now occurs, and he predicted a new element would be found which would have the general properties now attributed to the rare earths. Shortly afterwards (1879) scandium was discovered, and its discovery greatly aided in the general acceptance of Mendeleyev s ideas. While the chart had a place for lanthanum, there was no place in his chart for the other rare earths, since they also seemed to fall in the space reserved for lanthanum. The early chemists seemed to think they were discovering a new type of element with properties very similar to the properties which we now ascribe to isotopes, and some even speculated that these other rare earths were different modifications of lanthanum. 

The hydroxide and oxide phases that exist for scandium(III) include scandium hydroxide, Sc(OH)3(s), which likely has both amorphous and crystalline forms, ScOOH(s), and scandium oxide. It would be expected that the amorphous hydroxide is the most soluble and Sc203(s) the least. There have been few studies that have examined the solubility of these phases. None of the hydroxide or oxide phases of scandium are known to form naturally, with scandium present, as a major component, in eight mineral phases only (Wood and Samson, 2006). Moreover, there is much conjecture over which phases have actually been studied as well as their crystallinity (Baes and Mesmer, 1976 Wood and Samson, 2006). This review has selected data for both the hydroxide and oxide phases, based largely on the similarity in the stabilities obtained for the phases studied. Baes and Mesmer (1976) provided a stability constant for ScOOH(s) on the basis of data provided by Schindler (1963), but it is believed that the phase actually studied was Sc(OH)3(s) given the similar solubility found in the studies of Feitknecht and Schindler (1963) and Shkolnikov (2009). 

Ivanov-Emin, Nisel son andIvolgina (1960) andIvanov-Eminetal. (1968) studied the solubility of well-crystallised scandium hydroxide in solutions of sodium and potassium hydroxide. They showed that the solubility increased linearly with an increase in the hydroxide concentration. The stability of the solubility reaction should only be slightly dependent on ionic strength and medium and, as such, Ivanov-Emin et al. found a very similar stability constant in both bases. The constant obtained should be similar to that at zero ionic strength and the average is... 

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