Hydration of lanthanides

Amongst the known examples of this arrangement are a number of [M(H20)9] + hydrates of lanthanide salts and [ReH9] . The latter is... 

The direct measurement of enthalpies of solution of lanthanide trifluorides in water is hardly practicable, in view of their very small solubilities in this solvent. Enthalpies of hydration of lanthanide trifluorides have been measured (217). It should be possible to derive enthalpies of solution, at least for the hemihydrates LnF3-y2H20, by measuring enthalpies of precipitation. 

In this section are discussed hydrates of lanthanide salts where the water molecules form a majority of the coordination sphere and where there is evidence concerning the number or... 

The enthalpy of solution of anhydrous Lala in water is -201 kj mol S as already listed. Using the data discussed in the section on enthalpies of solution of hydrates of lanthanide chlorides in water, it can thence be deduced that the enthalpy of solution of a hexahydrate, Lal3-6H 0, would be between -100 and -120 kJ mol . Certainly the enthalpy of solution of Lals-GHjO in water would be much less negative than that of Lalg-ONHs in liquid ammonia. 

Hydration of lanthanide complexes. X-ray diffraction studies of the solid complexes of KLn(EDTA)(H20),c showed the number of water molecules in the coordination sphere to be three for the lighter lanthanides and two for the heavier ones for total coordination numbers of nine and eight, respectively, since EDTA is hexadentate (Hoard et al. 1967). Ots (1973) measured a maximum at europium for the heat capacity change AC° for the formation of lanthanide-EDTA complexes. This maximum was taken as a strong evidence for hydration equilibrium between the complexed species,... 

Rizkalla and Choppin (1991, 1994) have reviewed the hydration of lanthanide ions. They report that experimentally determined values (by electrophoresis and diffusion) for the hydrated radii of the lanthanides increase from La to Dy but i jparently level off for the heavier lanthanides (fig. 7). Replicate determinations by different authors place the uncertainty on these experimental values at 0.02-0.03 A. The apparent discontinuity near Tb is curious, but is paralleled by the heats and fiee energies of formation of the aquo cations. The similarity suggests that the observed trend represents a real chemical characteristic of the ions, perhaps related to the change in the inner-sphere coordination number or the balance of inner-sphere/second-sphere hydration. A simple analysis of the ions based on these hydrated radii indicates hydration numbers of 12-15 across the series (Lundqvist 1981). David and Fourest (1997) offer a more detailed interpretation that suggests a larger number of waters associated with the lanthanide cations. 

The experimental measurements that provide the hydration number and hydrated radius information are made on lanthanide solutions of moderate concentration with dilFerent counter ions. The data in Rizkalla and Choppin (1991) indicate that hydration numbers and Ln-O distances change slightly with both the nature of the counter ion and the concentration of the salt. It appears likely that composition of the primary coordination sphere of the lanthanide ion does not vary appreciably with the concentration (or identity of the counterion) of the lanthanide salts. However, the reduced water activity that occurs in concentrated salt solutions would suggest that overall hydration numbers will be higher in dilute solutions. Thus the values reported for overall hydration and hydrated radii determined in concentrated aqueous salt solutions probably underestimate the hydration of lanthanide cations in the dilute solutions that are typical of analytical applications. It has been suggested that as many as 40 water molecules may feel the presence of a trivalent lanthanide ion in solution (Choppin 1997). Using Lundqvist s (1981) estimate of 30 for the volume of a water molecule, the radial distance of the lanthanide iQrdration sphere... 

The chlorides, bromides, nitrates, bromates, and perchlorate salts ate soluble in water and, when the aqueous solutions evaporate, precipitate as hydrated crystalline salts. The acetates, iodates, and iodides ate somewhat less soluble. The sulfates ate sparingly soluble and ate unique in that they have a negative solubitity trend with increasing temperature. The oxides, sulfides, fluorides, carbonates, oxalates, and phosphates ate insoluble in water. The oxalate, which is important in the recovery of lanthanides from solutions, can be calcined directly to the oxide. This procedure is used both in analytical and industrial apptications. 

Figure 30.3 Variation with atomic number of some properties of La and the lanthanides A, the third ionization energy (fa) B, the sum of the first three ionization energies ( /) C, the enthalpy of hydration of the gaseous trivalent ions (—A/Zhyd)- The irregular variations in I3 and /, which refer to redox processes, should be contrasted with the smooth variation in A/Zhyd, for which the 4f configuration of Ln is unaltered.

Vikram L, Sivasankar BN (2008) New nine coordinated hydrated heavier lanthanide ethyl-diamine tetraacetates containing hydrazinium cation Crystal structure of N2H5[Dy(EDTA) (H20)3(H20)5. Ind J Chem 47A 25-31... 

Rowley, A. T. et al., Inorg. Chem. Acta, 1993, 211(1), 77 Preparation of metal oxides by fusing metal halides with lithium oxide in a sealed tube leads to explosions if halide hydrates are employed, particularly lanthanide trihalide hydrates. The preparation succeeds with anhydrous halides. This will be purely a question of vapour pressure above an exothermic reaction the question is whether the vapour is water, or metal halide, and the reaction oxide formation, or hydration of lithium oxide. Like other alkali metal oxides, hydration is extremely energetic. 

The hydration state of lanthanide(III) chelates can be assessed by different techniques. Luminescence studies are widely used for Eu111 and Tb111 chelates (see Chapter 9.21).17 18 170 NMR chemical shift measurements in solution of lanthanide(III) (most often Dy or Gd) complexes can also give information on q.19 These techniques in the context of MRI contrast agent research have been reviewed in 2001.1... 

One of the consequences of the lanthanide contraction is that some of the +3 lanthanide ions are very similar in size to some of the similarly charged ions of the second-row transition metals. For example, the radius of Y3+ is about 88 pm, which is approximately the same as the radius of Ho3+ or Er3 +. As shown in Figure 11.8, the heats of hydration of the +3 ions show clear indication of the effect of the lanthanide contraction. 

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

The hydration state of lanthanide(III) chelates can be assessed by different techniques which have been reviewed in details (1,2). 

Strictly, the solubilities of salt hydrates in nonaqueous solvents, and of lanthanide trichlorides in 97% ethanol, mentioned in Section V,B,2,a,... 

Dehydrating agents have commonly been employed in the preparation of lanthanide sulfoxide complexes from hydrated lanthanide salts. For example, dimethoxypropane has been used to prepare both (CH2)4SO (65) and rePr2SO (56) complexes of the lanthanide nitrates. An alternative dehydrating agent is ethyl orthoformate [Eq. (14)]. 

A number of complexes of urea and substituted ureas with various lanthanide salts have been isolated. The lanthanide acetates give both anhydrous and hydrated complexes with urea (67, 68). The hydrated complexes could be dehydrated by drying the complexes over CaCl2 or P4Oi0 (68). It is interesting to note that in the complexes of substituted ureas like EU (70) and CPU (71), the L M is independent of the anion. The anions in these complexes with a L M of 8 1 are apparently nonco-ordinated. Seminara et al. (72) have reported complexes of lanthanide chlorides with DMU and DEU which contain five and three molecules of the ligand respectively per... 

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.)...

However, even this simplified formula does not justify the use of the ratio of stability constants of the extracted complexes as the only measure of selectivity of extractive separations. Such a widely used approach is obviously based on an implicit assumption that the partition constants of neutral complexes ML of similar metal ions are similar, so that their ratio should be close to unity. This is, however, an oversimplification because we have shown that the ifoM values significantly differ even in a series of coordi-natively saturated complexes of similar metals [92,93]. Still stronger differences in the values have been observed in the series of lanthanide acetylacetonates, due to different inner-sphere hydration of the complexes (shown earlier), but in this case, self-adduct formation acts in the opposite direction [100,101] and partly compensates the effect of the differences in. Tdm on S T(see also Fig. 4.15). Such compensation should also be observed in extraction systems containing coordinatively unsaturated complexes and a neutral lipophilic coextractant (synergist). 

The enthalpies of hydration of the lanthanides are given in Table 8.4 and show a regular increasing negative value with decreasing ionic radius. 

The enthalpies of hydration of the Ln ions were calculated and their relationship to the lanthanide contraction was discussed. 

The carbonato complexes of lanthanides and actinides are of importance with regard to the metal ion speciation in the environment. These are, however, not linked with the enzyme models for carbon dioxide hydration and hence are not dealt with in further detail. 

The study of coordination compounds of the lanthanides dates in any practical sense from around 1950, the period when ion-exchange methods were successfully applied to the problem of the separation of the individual lanthanides,131-133 a problem which had existed since 1794 when J. Gadolin prepared mixed rare earths from gadolinite, a lanthanide iron beryllium silicate. Until 1950, separation of the pure lanthanides had depended on tedious and inefficient multiple crystallizations or precipitations, which effectively prevented research on the chemical properties of the individual elements through lack of availability. However, well before 1950, many principal features of lanthanide chemistry were clearly recognized, such as the predominant trivalent state with some examples of divalency and tetravalency, ready formation of hydrated ions and their oxy salts, formation of complex halides,134 and the line-like nature of lanthanide spectra.135... 

Russian workers have continued to study hydrated salts of lanthanide ions. Some of these studies are of a rather detailed nature and a summary of some of the more recent work will be given here to indicate its general nature. There is considerable emphasis on phase studies, X-ray powder data, thermogravimetric and differential thermal analysis, and IR data. 

It will be seen from Table 5 that the variation of parameters with atomic number is by no means always monotonic. The explanation of this must again be connected with variation of coordination number along the series of lanthanides. If the strongly coordinating organic ligands are reasonably assumed to exert their full ligancy, then any variation must be in the extent of inner-sphere hydration of the various species, as it is rather unlikely that there is a... 

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