Lanthanide hydrates

It has always been assumed that the hydration numbers for the lanthanides are higher than six, probably between 8 and 10, in analogy with the presence of enneaaquo ion [M(OH2)9 +] in neodymium bromate and ethylsulphate (see later, p. 121). Lanthanide hydration numbers have not been rigorously established, but some attempts have been made to study the problem by NMR technique (13—15). It is rather unfortunate that only low value for the hydration numbers ( 6) have been obtained, except for Er(III) and Yb(III) ions (756), where the hydration number is seven. 

Metal-oxygen bond distances in solid lanthanide hydrates. 

Data on the interesting Schottky transformations in the lanthanide hydrated ethyl sulfates by Meyer and his collaborators (89,138, 140) are suggestive and of considerable interest, but they do not cover a sufficiently extended range to permit entropy calculations. Vapor pressure data of metallically bonded lanthanide-magnesium alloys have been used also to deduce enthalpies of formation of CsCl-type compounds. Since data were unavailable, ACp values were assumed to be zero (147). 

Based on the data reviewed in this chapter, we draw the following general conclusions about lanthanide hydration and hydrolysis. 

Metal-oxygen distances in TCTP lanthanide hydrates. 

Lanthanide hydration data obtained from diffraction measurements. 

Lanthanide solvates with DMF and other amides have been known for more than twenty-five years. Krishnamurthy and Soundararajan (1966) have isolated neutral adducts, [Ln(DMF)4(N03)3] (Ln=La, Pr, Sm, Y), which dissociate in DMF solution to give 1 1 electrolytes. Lanthanide perchlorates form 8-coordinate solvates with DMF and 6-coordinate complexes with diphenylamide (Krishnamurthy and Soundararajan 1969). A calorimetric study of the reaction between lanthanide-hydrated triflates and DMF in ethanol has been carried out by Ziimer et al. (1991). As in the case of DMSO, numerous spectroscopic studies (absorption, luminescence) have been conducted on lanthanide ions in DMF solutions (cf. Legendziewicz et al. 1985, Jezowska-Trzebiatowska et al. 1984, Lugina et al. 1973). These are not reviewed here. 

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. 

A soluble sodium tripolyphosphate is produced as are iasoluble lanthanide and thorium hydroxides (hydrated oxides). 

Production of Cerium Derivatives. Moderately pure (90—95%) cerium compounds can be made from rare-earth chloride through oxidation with, for example, hypochlorite to produce an iasoluble cerium hydrate. The other lanthanides remain ia solutioa. The hydrate, oa calciaatioa, coaverts to Ce02. 

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

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.

The coordination chemistry of the large, electropositive Ln ions is complicated, especially in solution, by ill-defined stereochemistries and uncertain coordination numbers. This is well illustrated by the aquo ions themselves.These are known for all the lanthanides, providing the solutions are moderately acidic to prevent hydrolysis, with hydration numbers probably about 8 or 9 but with reported values depending on the methods used to measure them. It is likely that the primary hydration number decreases as the cationic radius falls across the series. However, confusion arises because the polarization of the H2O molecules attached directly to the cation facilitates hydrogen bonding to other H2O molecules. As this tendency will be the greater, the smaller the cation, it is quite reasonable that the secondary hydration number increases across the series. 

Hydrothermal hydrolysis of metal ions is useful in producing crystalline phases which contain metals in a state of partial hydrolysis, i.e., a state intermediate between that of the hydrated metal ion and that of the hydrous hydroxide. Such reactions have been used to produce numerous crystalline phases of actinides (1-4), Group IV metal ions (5-14) and lanthanides (15-21). 

The indium-mediated allylation of trifluoroacetaldehyde hydrate (R = H) or trifluoroacetaldehyde ethyl hemiacetal (R = Et) with an allyl bromide in water yielded a-trifluoromethylated alcohols (Eq. 8.56).135 Lanthanide triflate-promoted indium-mediated allylation of aminoaldehyde in aqueous media generated (i-airiinoalcohols stereoselectively.136 Indium-mediated intramolecular carbocyclization in aqueous media generated fused a-methylene-y-butyrolactones (Eq. 8.57).137 Forsythe and co-workers applied the indium-mediated allylation in the synthesis of an advanced intermediate for azaspiracids (Eq. 8.58).138 Other potentially reactive functionalities such as azide, enone, and ketone did not compete with aldehyde for the reaction with the in situ-generated organo-indium intermediate. 

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... 

Ferenc, W. et al., Monatsh. Chem., 1987, 118, 1087-1100 Preparation of the 2-nitrobenzoate salts of yttrium and the lanthanide metals (except praseodymium) as mono- or di-hydrates was studied. All melted and decomposed explosively above 250°C. 

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... 

Figure 7 Luminescent lanthanide complexes with representative luminescence lifetimes, and hydration states (derived from luminescence measurements) where appropriate.

---