Lanthanide trihalides structures

The lanthanide trihalides structures are known to exhibit a high frequency of nine-coordinate-cation geometries (Karraker, 1970) but only recently has an... 

The structure and vibrational spectra of the lanthanide trihalides have been studied extensively in the past decade. Due to the recent developments in the experimental and theoretical... 

This volume of the Handbook illustrates the rich variety of topics covered by rare earth science. Three chapters are devoted to the description of solid state compounds skutteru-dites (Chapter 211), rare earth-antimony systems (Chapter 212), and rare earth-manganese perovskites (Chapter 214). Two other reviews deal with solid state properties one contribution includes information on existing thermodynamic data of lanthanide trihalides (Chapter 213) while the other one describes optical properties of rare earth compounds under pressure (Chapter 217). Finally, two chapters focus on solution chemistry. The state of the art in unraveling solution structure of lanthanide-containing coordination compounds by paramagnetic nuclear magnetic resonance is outlined in Chapter 215. The potential of time-resolved, laser-induced emission spectroscopy for the analysis of lanthanide and actinide solutions is presented and critically discussed in Chapter 216. 

G. Lanza, Z. Varga, M. Kolonits, M. Hargittai, On the Effect of 4f Electrons on the Structural Characteristics of Lanthanide Trihalides. Computational and Electron Diffraction Study of Dysprosium Trichloride. J. Chem. Phys. 2008, 128, 074301-1-14. 

The lanthanide trihalides demonstrate very clearly the effect of varying the cation and anion radii upon the structure type adopted (Table 3.1). 

The actinide trihalides display a similar pattern of structure to those of the lanthanide trihalides. However, comparing the coordination numbers for Ln + and An + ions with the same number of f electrons ( above one another in the Periodic Table ), it can be seen that the coordination number of the lanthanide halides decreases sooner than in the actinide series, a reflection of the fact that the larger actinide ions allow more halide ions to pack around them. Table 10.3 gives comparative coordination numbers for the trihalides of the lanthanides and actinides. 

It should be noted here that not all these synthetic routes are equally well applicable to the rare earth elements. Route (b) is severely restricted by the paucity of simple lanthanide alkyls, while route (d) is unsuitable in the lanthanide case as lanthanide trihalides are generally unreactive towards Al,Al,Al -tris(trimethylsilyl)amidines. Deprotonation of amidines by metal amides should also be possible synthetic pathway since the delocalized structure of the resulting anion will increase the acidity. However, this route has apparently not yet been tried. 

An electrostatic hydration model, previously developed for ions of the noble gas structure, has been applied to the tervalent lanthanide and actinide ions. For lanthanides the application of a single primary hydration number resulted in a satisfactory fit of the model to the experimentally determined free energy and enthalpy data. The atomization enthalpies of lanthanide trihalide molecules have been calculated in terms of a covalent model of a polarized ion. Comparison with values obtained from a hard sphere modeP showed that a satisfactory description of the bonding in these molecules must ultimately be formulated from the covalent perspective. 

Joubert et al presented a systematic study of the structural and bonding properties of selected lanthanide trihalide molecules, LnXa (Ln = La, Gd, Lu X = F, Cl). An ELF analysis revealed typical ionic bonding properties, emphasizing the increasing ionic character of the Ln-X bonds through the rare-earth series. Moreover the authors pointed out a strong distortion of the outer core shell of the metal. 

Some relevant data are compiled in table 4 and the enthalpies of solution are plotted in fig. 6. We note that, as is the case for sesquioxides, all enthalpies of solution of actinide trihalides are less exothermic than those of structurally similar lanthanide trihalides. See section 4.1.1 for further discussion. 

It has been noted (Morss 1986, fig. 17.10 this chapter, section 4.1.1 and fig. 3) that the actinide trihalides have less exothermic heats of solution than structurally similar lanthanide trihalides. 

The lanthanide and actinide halides remain an exceedingly active area of research since 1980 they have been cited in well over 2500 Chemical Abstracts references, with the majority relating to the lanthanides. Lanthanide and actinide halide chemistry has also been reviewed numerous times. The binary lanthanide chlorides, bromides, and iodides were reviewed in this series (Haschke 1979). In that review, which included trihalides (RX3), tetrahalides (RX4), and reduced halides (RX , n < 3), preparative procedures, structural interrelationships, and thermodynamic properties were discussed. Hydrated halides and mixed metal halides were discussed to a lesser extent. The synthesis of scandium, yttrium and the lanthanide trihalides, RX3, where X = F, Cl, Br, and I, with emphasis on the halide hydrates, solution chemistry, and aspects related to enthalpies of solution, were reviewed by Burgess and Kijowski (1981). The binary lanthanide fluorides and mixed fluoride systems, AF — RF3 and AFj — RF3, where A represents the group 1 and group 2 cations, were reviewed in a subsequent Handbook (Greis and Haschke 1982). That review emphasized the close relationship of the structures of these compounds to that of fluorite. 

Since the structural modification adopted by the trihalides changes systematically with ionic radius and since the relatively soft halide ions undergo compression rather easily, lanthanide halide structures are pressure sensitive. Most lanthanide, numerous actinide, and several mixed lanthanide/actinide systems have been examined under elevated pressures, the typical effect of which is transformation to a more densely packed structure. 

Ah initio calculations of vibrational wavenumbers for lanthanide trihalides suggested some re-assignments of modes for ScBrs, YF3 and YCI3. Raman spectroscopy was used to characterise the solution-phase structures of [Ln WioOse] , where Ln = Y, La, Ce, Pr, Sm, Eu, Gd, Dy, Er or Lu, and [M WioOse], where M = Ce or Th. ... 

According to Figure 21, we should not expect noticeable changes in the ratio between different structural units in melts of lanthanide trihalides that have identical structures in the solid state. A noticeable change in this ratio is likely characteristic of melts of lanthanide trichlorides only in going from hexagonal to monoclinic structures. 

Pressure-temperature-induced structural transitions to the PuBrj structure type in lanthanide trihalides, determined on quenched samples by Beck and Gladrow (1979). 

The melting points in all four lanthanide trihalide series exhibit a minimum which shift from the heavy lanthanides to the light lanthanides as the atomic number of the halide increases. In the fluoride series the minimum occurs at ErFs, in the chloride series at TbCb, in the bromide series probably at SmBrs, and in the iodide series at Prl3. This minimnm concurs with a change in the stable crystallographic structure, except for the bromide series in which the high temperature polymorphic behaviour is not knowm... 

A. Kovacs and R. J. M. Konings, Structure and vibrations of lanthanide trihalides An assessment of experimental and theoretical data, J. Phys. Chem. Ref. Data, 33,377-404 (2004). M. Dolg, Segmented contracted Douglas-Kroll-Hess adapted basis sets for lanthanides,... 

Physics 2008, 128, 74301. (d) Topological approach in the structural and bonding characterization of lanthanide trihalide molecules, L. Joubert, B. SUvi, G. Picard, Theoretical Chemistry Accounts 2000, 104, 109. 

We now turn to the 3d series elements. The dihalides and trihalides can be treated as ionic solids, although the chlorides, bromides and iodides adopt layer structures which might be better viewed as polymeric covalent crystals. In Fig. 5.2 the third ionisation energies of the 3d atoms are plotted alongside those of the lanthanides. These all involve the removal of an electron from a 3d orbital from Fe onwards, the orbital concerned is doubly occupied so that spin-pairing energy assists the ionisation. This accounts for the break between Mn and Fe, as previously discussed (Section 4.3). The increase from Sc to Mn, and from Fe to Zn, is much sharper than the corresponding increases in the lanthanide series. However, the break at the half-filled shell is less abrupt for the 3d series. This explains why the II oxidation state - which is... 

The crystal structures of the various lanthanide halides into which einsteinium trihalides were incorporated are summarized in Table IV. Both bromides and chlorides were prepared. The crystal... 

The trihalides MBr3 and MI3 are known for all the lanthanide elements. The early lanthanide tribromide (La to Pr) adopt the LaCh structure, while the later tribromides (from Nd to Lu) and the early triiodides (from La to Nd) form a layer structure with eight-coordinate lanthanide ions. 

Trihalides are known for most of the actinides and form the basis for comparison with the lanthanides. Table 10.2 lists the known structures adopted by the trihalides. 

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