Lanthanides lightest

A-type, eonsisting of LnOy units whieh approximate to eapped oetahedral geometry, and favoured by the lightest lanthanides. 

Fig. 4.15 The system La(III) acetylacetone (HA) - IM NaC104/benzene at 25°C as a function of lanthanide atomic number Z. (a) The distribution ratio Hl (stars, right axis) at [A ] = 10 and [HA] rg = 0.1 M, and extraction constants (crosses, left axis) for the reaction Ln + 4HA(org) LnA3HA(org) + 3FE. (b) The formation constants, K , for formation of LnA " lanthanide acetylacetonate complexes (a break at 64Gd is indicated) circles n = 1 crosses n = 2 triangles w = 3 squares w = 4. (c) The self-adduct formation constants, for the reaction of LnA3(org) + HA(org) LnA3HA(org) for org = benzene. (A second adduct, LnA3(HA)2, also seems to form for the lightest Ln ions.) (d) The distribution constant Ajc for hydrated lanthanum triacetylacetonates, LnAs (H20)2 3, between benzene and IM NaC104. (From Ref. 28.)...

ATdc values for the lanthanide acetylacetonates are the reverse from that expected for the size effect. The explanation is hkely that the neutral complex with the formula LnAs is coordinatively unsaturated, which means that a hydrated complex exists in the aqueous phase (possibly also in the organic phase). The more coordinatively unsaturated lanthanide complexes (of the larger ions) can accommodate more water and thus are more hydrophilic. The result is a A dc several orders of magnimde lower for the lightest. La, than for the heaviest, Lu. 

Yttrium is always found with the rare-earth elements, and in some ways it resembles them. Although it is sometimes classified as a rare-earth element, it is listed in the periodic table as the first element in the second row (period 5) of the transition metals. It is thus also classified as the lightest in atomic weight of all the rare-earths. (Note Yttrium is located in the periodic table just above the element lanthanum (group 3), which begins the lanthanide rare-earth series. 

It is interesting to compare the extraction of different lanthanides from acidic medium (1.5 M HN03) by OOCMPO at a concentration 0.25 M, 250 times higher than that of CPw3 or that of tetramer 054, the acyclic analogue of CPw3. For the lightest... 

Under the same conditions, in contrast to what is observed for calix[4]arene-bearing CMPO moieties, with CPil2, distribution ratios of lanthanides increase from the lightest lanthanide, lanthanum, to europium. Americium can be easily separated from the lightest lanthanides (separation factor DAm/La > 20, DAm/Ce =15, /lAlll,Nd = 10, UAi /si = 7.5, DAm/Eu = 6), which are the most abundant lanthanides in fission-product solution. Cavitands bearing picolinamide (Cv5) or thiopicolin-amide (Cv6) residues seems much less selective than their calixarene counterparts, giving SAm/Eu < 2.18... 

The type-A (hexagonal) structure consists of MO7 units which approximate to capped octahedral geometry, and is favored by the lightest lanthanides (La, Ce, Pr, and Nd). Figure 18.2.1(a) shows the structure of La2C>3. 

Table 2. Transition metal ions in doped halide lattices for which upconversion luminescence has been demonstrated, including relevant mechanistic and electronic-structural information. The lightest and heaviest lanthanides showing single-ion upconversion are also listed. Adapted from [17] ...

Diffraction methods. Bombardment of aqueous solutions of electrolytes by neutrons or X-rays causes scattering which is characteristic of the microscopic structure of the system. X-rays are preferentially scattered by heavy atoms whereas neutrons are scattered best by the lightest atoms. Direct determination of the number and geometry of the water molecules in the primary hydration sphere of the lanthanides have been attempted by both techniques. 

Lanthanide distribution coefficients were determined by Shimizu and Kushiro (1975) on a synthetic garnet. The garnet was separated from the quenched parent liquid for measurement of lanthanide concentrations by isotope dilution mass spectrometry. Values for the heavier lanthanides (fig. 21.20) are not appreciably disturbed by possible contamination by quenched liquid, but those for the lightest lanthanides may be substantially too high. 

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