Lanthanide fluoride

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. 

The existence of solvates of lanthanide halides may be taken as an indication of favorable interaction between the halides and solvent in question. Evidence of this type may suggest solvents which could prove useful or interesting for calorimetric or solubility studies. However, the existence of stable solvates is no guarantee of marked solubility for the halide in the solvent in question. One need look no further than lanthanide fluorides and water for cautionary examples, for despite their very low solubility in water these fluorides form a series of stable hemihydrates, LnF3-V H20. 

The lanthanide fluorides Eire all sparingly soluble in water, as are the fluorides of yttrium and scEmdiiun. Thus, solubility information is generally presented in the form of the solubility product Ksp. 

This expression has been written in terms of concentration if activity coefficients sue known or estimated, then a thermodynamically ideal solubility product may be obtained from the Emalogous product of ionic activities. As the concentration of ions in solutions of lanthanide fluorides is low, the concentration and activity solubility products will not differ markedly, although activity coefficients for these salts of 3 + cations are significantly less than unity even in such dilute solutions (4a). 

There have been many determinations of solubility products for lanthanide fluorides over the years. We shall try to show the range of values presented for a given element, as well as the chemically important variation of solubility with the nature of the metal. Values for aqueous solution will be reviewed first, then the effects of added acid or salts on solubility considered. 

Hexafluorophosphate derivatives, such as [M(Me2SO)n][PF6]3 (M = La, Lu, Y), have been synthesized (365), and infrared data show O-bonding of the sulfoxide with ionic hexafluorophosphate groups. Analytical data are incomplete for this series, as decomposition, postulated to be to lanthanide fluorides, occurs. The (CH2)4SO complexes [M( CH2 4SO)t.5][PF6]3 are reported (145), which are 3 1 electrolytes with uncoordinated anions, implying a possible semibridged structure, as previously mentioned. Complexes of other cyclic sulfoxides, including thioxane oxide (146) and trans-1,4-dithiane-1,4-dioxide (147) derivatives of hexafluorophosphate salts have also been prepared. 

The lattice contribution in the lanthanide fluorides is only known with sufficient accuracy when the f shell of the metal ions is empty (4f°) or completely filled (4f14). In these cases Sexs is zero and the experimental entropy corresponds to Siat. Also in case the f-shell of the metal... 

The properties of the liquid lanthanide trihalides depend strongly on the atomic number of the halide. The variation in the heat capacity of the lanthanide fluorides indicates a strongly ionic behaviour of the melts with a concomittent irregular trend related to the electronic configuration of the lanthanide ions. In the lanthanide chlorides, bromides and iodides the trend becomes systematically more constant, indicating an increasing molecular nature of the melts. 

The metals have similar reactivities to magnesium. This means that they cannot be extracted by methods like carbon reduction of the oxides in practice, metallothermic reduction of the lanthanide fluoride or chloride with calcium at around 1450 °C is used. The product... 

The early lanthanide fluorides adopt the LaFs structure (Figure 3.3) based on a metal ion surrounded by a trigonal prism of fluorides with two additional capping fluorides, giving 11 coordination (9 + 2), whilst the later fluorides have the YF3 structure. This is based on tricapped trigonal prismatic 9 coordination, in which the prism is somewhat distorted. The structure adopted by UCI3 and several of the lanthanide halides is again a tricapped... 

Eermium coprecipitates with lanthanide fluorides and hydroxides, showing it to form lanthanide-like Em + ions. These elute from cation-exchange resins slightly before Es +, whilst its chloride and thiocyanate complexes are eluted from anionic exchange resins just... 

For the trivalent and quadrivalent actinide and lanthanide fluoride complexes, Choppin and Unrein (57) suggest an equation for AGO which contains z+z /(r]yj + rp)e, with e the adjustable parameter. They give e = 79.8, 57.0, and 40.8 for MF" , MF " , and MF " " complexes. Introducing these values in the electrostatic or Fuoss model equations greatly improves the agreement between predicted and empirical data. However, it must be remembered that these e values also include the effect of decreasing d values in the complex with increased valence of the cation. 

These values compare favorably with those reported by Kury at /x = 0.5 and 25 °C. The stability constants of the lanthanide fluoride complexes were obtained by comparing changes in the potential in cell C with those in that of cell B upon adding sodium fluoride solution. Owing to the insolubility of LnFg, only a few (often only six) aliquots of sodium fluoride solution could be added to cell C before precipitation interferred. Precipitation could be observed easily since upon its occurrence, the potential between cells C and A would drift rapidly to lower values. 

Sun XQ, Peng B, Ji Y et al (2008) Ionic liquids based all-in-one synthesis and photoluminescence properties of lanthanide fluorides. J Phys Chem C 112 10083-10088... 

Alexander D. Chervonnyi, Thermodynamic Properties of Lanthanide Fluorides and Chlorides in the Gaseous and Condensed States 165... 

The synthesis of lanthanide and actinide compounds was the subject of a book (Meyer and Morss 1991). Detailed information is given on the synthesis of lanthanide fluorides (Muller 1991), binary lanthanide halides, RX3 (X=Cl,Br,l) (Meyer 1991a), complex lanthanide(in) chlorides, bromides and iodides (Meyer 1991b), and on two alternative routes to reduced halides, the conproportionation route (Corbett 1991) and the action of alkaU metals on lanthanide(lll) halides (Meyer and Schleid 1991). Therefore, a brief outline of the main preparative routes and synthetic strat es might be sufficent. 

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