Lanthanide ionic radius

Figure 5.3 The structure of [La(al-P2Wn06i)2] [13]. (Reprinted with permission from L. Fiesel-mann, et al., Influence of steric and electronic properties of the defect site, lanthanide ionic radii, and solution conditions on the composition of lanthanide(III) al-P2Wi70gj polyoxometalates, Inorganic Chemistry, 44, no. 10, 3569-3578 (Figure 2), 2005. 2005 American Chemical Society.)...

From Cantrell (1988), estimated from correlations of lanthanide ionic radii (Shannon, 1976) versus lanthanide carbonate complexation constants, plus the actinide ionic radius estimates of Shannon (1976). 

Fig. 7.5 Behaviour of L11C0O3 perovskites (a) H2 ( ) and CO (O) yields from partial oxidation of methane as a function of the lanthanide ionic radii and (b)Goldschmidt s tolerance factor t and the oxidation temperature obtained from temperature-programmed oxidation experiments (after Ref. 108).

While the idea that ligand 7 could prove useful for the coordination of other, non-uranyl actinide cations, has yet to be tested by experiment, it is important to note that this ligand has so far proved less than satisfactory for the coordination of other large, non-actinide cations. Indeed, in spite of extensive efforts devoted to the problem, no stable, non-labile complexes of the trivalent lanthanides (ionic radii 0.86 - 1.36 A ) have as yet been characterized with this system. Nor have 1 1 complexes with other large cations, e.g., Cd + (ionic radius 1.0 A ) or Pb + (ionic radius 1.2 A ), been documented.This has proven to be the case even though mass spectrometric evidence consistent with metal coordination has been obtained in certain instances. 

Table 2.1-25D Lanthanides. Ionic radii (determined from crystal structures)...

Fig. 13.19. Lanthanide ionic radii vs. reciprocal effective nuclear charge (Z r).

Where the lanthanide ionic radius and the macrocyclic cavity are incompatible, though in hydrous conditions the crown ether is likely to be displaced by water ligands, the crown may still be present in the structure of the crystal as a hydrogen-bonded adduct. This behaviour is seen in [Gd(N03)3(H20)3]-(18-crown-6).445 This type of compound is quite well known in the case of s block metals also, e.g. [Mg(H20)6]Cl2 (12-crown-4)454 and, a more subtle case, [Ca(nitrobenzoate)2(benzo-15-crown-5)]-3H20(benzo-15-crown-5)455 in which an apparent 2 1 complex has only half its crown ligand coordinated to Ca2+. 

Fig. 6. Plot of association constants for Ln " " and [Ln(D02A)]" to DPA against lanthanide ionic radius, 0.2 M NaOAc, pH 7.5. The addition of D02A enhances dipicolinate binding affinity by an order of magnitude for most lanthanides investigated and by nearly two orders of magnitude for terbium (light gray).

Figure 1.39 Generic structure of tf (dipicolinato)lanthanide complexes (top) (a) plot of the Ln-N (circle) and Ln-O (triangle) distances vs lanthanide ionic radius (h) absorption spectra in water plot of the hyperpolarisability coefficient /3 vs ionic radius (c) or /-orbital filling (d)...

It was found that the highest turnover frequency corresponds to Sm and decreases in parallel with the lanthanide ionic radius. All the catalysts screened display high selectivities for the formation of products with relative trans configurations (29a and 30a in Figure 6.1). Despite the potential enantios-electivity of the catalysts, no attempts to quantify the ratios or separate individual enantiomers were reported. 

Fig. 39.6. Enzyme activity as a function of lanthanide ionic radius. Filled circles relative biosynthetic activity of adenylated glutamine synthetase, data from Wedler and D Aurora (1974). Open circles reciprocal half-times for the conversion of trypsinogen to trypsin, data from Gomez et al. (1974).

The reason why lanthanides of high atomic number emerge first is that the stability of a lanthanide ion-citrate ion complex increases with the atomic number. Since these complexes are formed by ions, this must mean that the ion-ligand attraction also increases with atomic number, i.e. that the ionic radius decreases (inverse square law). It is a characteristic of the lanthanides that the ionic radius... 

Chemical Properties. Although the chemical properties of the trivalent lanthanides are quite similar, some differences occur as a consequence of the lanthanide contraction (see Table 3). The chemical properties of yttrium are very similar too, on account of its external electronic stmcture and ionic radius. Yttrium and the lanthanides are typical hard acids, and bind preferably with hard bases such as oxygen-based ligands. Nevertheless they also bind with soft bases, typicaUy sulfur and nitrogen-based ligands in the absence of hard base ligands. 

A full discussion of thorium electrochemistry is available (3). Thorium is generally more acidic than the lanthanides but less acidic than other light actinides, such as U, Np, and Pu, as expected from the larger Th" " ionic radius (108 pm). 

Scandium is very widely but thinly distributed and its only rich mineral is the rare thortveitite, Sc2Si20v (p. 348), found in Norway, but since scandium has only small-scale commercial use, and can be obtained as a byproduct in the extraction of other materials, this is not a critical problem. Yttrium and lanthanum are invariably associated with lanthanide elements, the former (Y) with the heavier or Yttrium group lanthanides in minerals such as xenotime, M "P04 and gadolinite, M M SijOio (M = Fe, Be), and the latter (La) with the lighter or cerium group lanthanides in minerals such as monazite, M P04 and bastnaesite, M C03F. This association of similar metals is a reflection of their ionic radii. While La is similar in size to the early lanthanides which immediately follow it in the periodic table, Y , because of the steady fall in ionic radius along the lanthanide series (p. 1234), is more akin to the later lanthanides. 

Figure 30.2 Variation of metal radius and 3+ ionic radius for La and the lanthanides. Other data for Ln" and Ln" are in Table 30.2.

By contrast, the ionic radius in a given oxidation state falls steadily and, though the available data are less extensive, it is clear that an actinide contraction exists, especially for the -f3 state, which is closely similar to the lanthanide contraction (see p. 1232). 

Actinium is similarly easy to find a proxy for. It forms large trivalent cations with an ionic radius of 1.12 A in Vl-fold coordination. This is somewhat larger than La (1.032 A), which we will adopt as a proxy. The partitioning behavior of the lanthanides (denoted collectively Ln) is sufficiently well understood to make this a prudent choice. In some minerals, however, the larger size of Ac, may place it onto a larger lattice site than the lanthanides. This possibility should be considered for minerals with very large cation sites, such as amphiboles and micas. 

Water exchange on [Ln(H20)8]3+ for the heavier lanthanides Gd3+-Yb3+ is characterized by a systematic decrease in /feHa0 and an increase in AH as the ionic radius decreases from Tb3+ to Yb3+, and both A Si and AV-t are negative (311-313). The AV are significantly less than either the value of -12.9 cm3 mol-1 calculated for water... 

At the first step, the insertion of MMA to the lanthanide-alkyl bond gave the enolate complex. The Michael addition of MMA to the enolate complex via the 8-membered transition state results in stereoselective C-C bond formation, giving a new chelating enolate complex with two MMA units one of them is enolate and the other is coordinated to Sm via its carbonyl group. The successive insertion of MMA afforded a syndiotactic polymer. The activity of the polymerization increased with an increase in the ionic radius of the metal (Sm > Y > Yb > Lu). Furthermore, these complexes become precursors for the block co-polymerization of ethylene with polar monomers such as MMA and lactones [215, 217]. 

Most lanthanide compounds are sparingly soluble. Among those that are analytically important are the hydroxides, oxides, fluorides, oxalates, phosphates, complex cyanides, 8-hydroxyquinolates, and cup-ferrates. The solubility of the lanthanide hydroxides, their solubility products, and the pH at which they precipitate, are given in Table 2. As the atomic number increases (and ionic radius decreases), the lanthanide hydroxides become progressively less soluble and precipitate from more acidic solutions. The most common water-soluble salts are the lanthanide chlorides, nitrates, acetates, and sulfates. The solubilities of some of the chlorides and sulfates are also given in Table 2. 

The overall distribution of lanthanides in bone may be influenced by the reactions between trivalent cations and bone surfaces. Bone surfaces accumulate many poorly utilized or excreted cations present in the circulation. The mechanisms of accumulation in bone may include reactions with bone mineral such as adsorption, ion exchange, and ionic bond formation (Neuman and Neuman, 1958) as well as the formation of complexes with proteins or other organic bone constituents (Taylor, 1972). The uptake of lanthanides and actinides by bone mineral appears to be independent of the ionic radius. Taylor et al. (1971) have shown that the in vitro uptakes on powdered bone ash of 241Am(III) (ionic radius 0.98 A) and of 239Pu(IV) (ionic radius 0.90 A) were 0.97 0.016 and 0.98 0.007, respectively. In vitro experiments by Foreman (1962) suggested that Pu(IV) accumulated on powdered bone or bone ash by adsorption, a relatively nonspecific reaction. On the other hand, reactions with organic bone constituents appear to depend on ionic radius. The complexes of the smaller Pu(IV) ion and any of the organic bone constituents tested thus far were more stable (as determined by gel filtration) than the complexes with Am(III) or Cm(III) (Taylor, 1972). 

FIGU RE 11.8 Heat of hydration of + 3 lanthanide ions as a function of ionic radius. 

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