Europium oxidation states

Lanthanide fluoride sequences show monotonic increase in bond energy with increasing oxidation state, although one would expect a maximum for EuF2 in the europium series at the stable f7 configuration. 

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... 

Morris R. V. and Haskin L. A. (1974). EPR measurement of the effect of glass composition on the oxidation states of europium. Geochim. Cosmochim. Acta, 38 1435-1445. 

Am3+ is the most stable oxidation state of the metal. In trivalent state, its properties are simdar to europium. Am3+ reacts with soluble fluoride, hydroxide, phosphate, oxalate, iodate and sulfate of many metals forming precipitates of these anions e.g., Am(OH)3, Am(103)3, etc. 

Trivalent europium is an excellent ionic probe for materials and its luminescence properties are extensively studied. Eu is one of the mostly informative elements in mineralogy, especially when the ratio Eu /Eu may be assessed. Both oxidation states are luminescent, but the hnes of Eu in minerals are usually very weak and concealed by other centers. By steady state liuninescence spectroscopy its luminescence has been confidently detected only in scheehte and anhydrite (Tarashchan 1978 Gorobets and Rogojine 2001). 

This chapter commences with a review of a limited number of ternary hydride systems that have two common features. First, at least one of the two metal constituents is an alkali or alkaline earth element which independently forms a binary hydride with a metal hydrogen bond that is characterized as saline or ionic. The second metal, for the most part, is near the end of the d-electron series and with the exception of palladium, is not known to form binary hydrides that are stable at room temperature. This review stems from our own more specific interest in preparing and characterizing ternary hydrides where one of the metals is europium or ytterbium and the other is a rarer platinum metal. The similarity between the crystal chemistry of these di-valent rare earths and Ca2+ and Sr2+ is well known so that in our systems, europium and ytterbium in their di-valent oxidation states are viewed as pseudoalkaline earth elements. 

Although all the lanthanides are stable in the solid state as M2+ ions doped into CaF2 crystals, only in the cases of europium, ytterbium and samarium is there any real coordination chemistry, and that is very limited. There is a small but developing organometallic chemistry of the lower oxidation states,641 but that is not within the scope of the present review. Much of the chemistry of the dipositive state depends on solvated species642 and it is convenient to begin with these. 

Solutions of alkali metals in ammonia have been the best studied, but other metals and other solvents give similar results. The alkaline earth metals except- beryllium form similar solutions readily, but upon evaporation a solid ammoniste. M(NHJ)jr, is formed. Lanthanide elements with stable +2 oxidation states (europium, ytterbium) also form solutions. Cathodic reduction of solutions of aluminum iodide, beryllium chloride, and teUraalkybmmonium halides yields blue solutions, presumably containing AP+, 3e Be2, 2e and R4N, e respectively. Other solvents such as various amines, ethers, and hexameihytphosphoramide have been investigated and show some propensity to form this type of solution. Although none does so as readily as ammonia, stabilization of the cation by complexation results in typical blue solutions... 

These compounds, tested in NPHE at Cadarache, were used as reference compounds for the extraction of actinides by functionalized calixarenes (see below). The distribution ratios for neptunium mainly at the oxidation state (V), plutonium at the oxidation state (IV), and americium (III) are shown in Table 4.21 for OOCMPO. They were also used as references for the americium over europium selectivity (Table 4.22). 

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... 

The most successful of the anion emitters are based on rare earth oxide matrices, with the rare earth in the +3 oxidation state. Europium oxide, Eu703, is the most successful of these anion emitters and also has the most stable +2 oxidation state of all the rare earths. This feature of Eu203, the stability of the +2 oxidation state, is thought to be responsible for this compound s being the best matrix. Whereas the matrix in the +3 oxidation state allows the migration of the anions away from the counter ion, the stability of the +2 oxidation states allows reactions of the following type to take place ... 

The rare earth must have a reasonably stable +2 oxidation state, although the majority of the material must be in the +3 state. The elements europium (Eu) and ytterbium (Yb) have by far the most stable +2 oxidation states of the rare earths, and the oxides of these elements make the most effective matrices for anion emission. Eu is approximately two orders of magnitude more effective as a matrix that Nd when perrhenate emission is not pushed to high levels. 

Xe-like electronic configuration is adopted. The + 2 oxidation state is most relevant for samarium (f6, near half-filled), europium (f7, half-filled), thulium (f13, nearly filled) and ytterbium (f14, filled). In order to attain the more stable + 3 oxidation state, Sml2 readily gives up its final outer-shell electron, in a thermodynamically driven process, making it a very powerful and synthetically useful single-electron transfer reagent. 

Addition of the same NHC to Eu(thd)3 (thd — tetramethylheptanedioate) affords the europium(III) adduct Eu(thd)3(NHC). The europium-NHC bond distance of 2.663(4) A is shorter than that of the samarium(II) complex and is consistent with the higher oxidation state of the lanthanide centre. The yttrium(III) analogue was also prepared and characterised by NMR spectroscopy. The C2 carbon resonates at 199 ppm in the 13C NMR spectrum, with a yc coupling constant of 33 Hz. This indicates that the NHC remains bound to the metal centre in solution and does not dissociate on the NMR timescale. 

Metallic uranium (180, 181) has a photo-electron spectrum with distinct signals, the strongest originating in the 4/, 4d and 5d shells. Most uranium(VI) compounds have similar spectra (15, 37, 181) with a modest chemical shift dl amounting to 3 to 5 eV. This behaviour is comparable to the d groups and very different from europium, where it was early shown (39) that metallic alloys containing the conditional oxidation state 4/7 Eu[II] and Eu(II) compounds such as EuSCU (15) have I of all the inner shells 10 eV lower than of the Eu(III) compounds. 

Europium(III) exchanged zeolites have been studied by a number of research groups. Arakawa and coworkers (20, 21 ) report the luminescence properties of europium(III)-exchanged zeolite Y. Emission spectra were measured under a variety of conditions and bands for europium(II) were observed after thermal treatment of the europium(III) Y zeolites. A mechanism was proposed for the thermal splitting of water which involved the cycling of europium between the two different oxidation states. Europium MSssbauer experiments (22 ) also show that on thermal treatment of europium-(III) zeolites that europium(II) is formed. Stucky and coworkers (23, 24) studied the phosphorescence lifetime of these europium-(lll) zeolites and showed that the inverse of the lifetime (the decay constant) was linearly related to the number of water molecules surrounding the europium(III) ion in the zeolite supercages. These studies involved zeolites A, X, Y and ZSM-5. 

In halides lower oxidation states are more stable than in oxides. In most oxide lattices europium and ytterbium are the only lanthanide ions which, because of... 

Consistent with the stability of the divalent oxidation states see Formal Oxidation State) of ytterbium and europium, the triflates of Yb(III) and Eu(III) are reduced by the bis(trimethylsilyl)allyl anion, leading to Ln(II) species (equation A) The same compounds can be made in higher yield by starting with Yb(II) and Eu(II) triflates directly. 

In aqueous solution, lanthanides are most stable in the tripositive oxidation state, making them difficult to separate and purify. The preference for this oxidation state is due in part to the energy of the 4f electrons being below those of the 5d and 6s electrons (except in the cases of La and Ce). When forming ions, electrons from the 6s and 5d orbitals are lost first so that all Ln + ions have [Xe] 4f electronic configurations. Under reducing conditions, certain lanthanides (europium, samarium, and ytterbium) can be stable as dipositive ions, and cerium can adopt a +4 oxidation state (5). 

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