Thermodynamics, lanthanide chemistry

Trends in the properties of lanthanides are usually visualized as Z plots although in some cases, plots against orbital angular momentum show linear relationships where the more traditional Z plots are difficult to interpret. The thermodynamic parameters required for a firm underpinning of much of lanthanide chemistry are now in place and most of the important quantities have been determined or reliably estimated. Revised ionic radii are now available and it will be interesting to see whether these replace the classical Shannon-Prewitt radii which have been used for over 30 years. 

Unlike the di-f dihalides, such compounds differ little in energy from both the equivalent quantity of metal and trihalide, and from other combinations with a similar distribution of metal-metal and metal-halide bonding. So the reduced halide chemistry of the five elements shows considerable variety, and thermodynamics is ill-equipped to account for it. All four elements form di-iodides with strong metal-metal interaction, Prl2 occurring in five different crystalline forms. Lanthanum yields Lai, and for La, Ce and Pr there are hahdes M2X5 where X=Br or I. The rich variety of the chemistry of these tri-f compounds is greatly increased by the incorporahon of other elements that occupy interstitial positions in the lanthanide metal clusters [3 b, 21, 22]. 

Further developments involve the investigation of the mechanism of formation of double- and triple helicates and of the effect of variations in ligand structure on their features, the determination of their physico-chemical (thermodynamic, kinetic, electrochemical, photochemical) properties, the exploration of the coordination chemistry of the ligand strands. For instance, it may be possible to obtain quadruple helical complexes with ions of high coordination number such as the lanthanides and linear ligands containing bipy or terpy units. Using cubic metal ions would also be of interest. 

Miguirditchian, M., Guillaneux, D., Guillaumont, D., Moisy, P., Madic, C., Jensen, M., Nash, K.L. 2005. Thermodynamic study of the complexation of trivalent actinide and lanthanide cations by ADPTZ, a tridentate N-donor ligand. Inorganic Chemistry 44(5) 1404-1412. 

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. 

In aqueous solutions, lanthanide(III) ions are coordinated by water molecules. The hydration sphere of the lanthanide ions plays a vital role in the chemistry of the ion and also in several biochemical reactions involving isomorphous calcium(II) substitution reactions. The interpretation of the absorption spectra of lanthanide(III) ions in aqueous media is difficult because of the variability of the coordination number of the aquo ions along the lanthanide series. Kinetic and thermodynamic studies [206-210] on the lanthanide aquo systems led to the conclusion that the lighter lanthanides have a coordination number of 9, heavy lanthanides are octacoordinated and the middle members exist in a equilibrium mixture of octa and nonacoordinated aquo ions. 

Figure 1.14 The relationship between the atomic numher of lanthanides and thermodynamic functions (Kex, AH, AZ°, and AS°) from the exaction system consisting of 2-ethyl hexyl mono(2-ethyl hexyl) ester phosphinate in a dodecane solution [14]. (Reprinted from E.X. Ma, X.M. Yan, S.Y. Wang, et al., The extraction chemistry of tanthanides with 2-ethyl-hexyle mono (2-ethyl-hexyle) phosphinate oxide, Scientia Sinica B Chemistry (in Chinese), 5, 565-573, 1981, with permission from Science in China Press.)...

Kremer, C., Torres, J., Dominguezb, S., and Mederos, A. (2005) Structure and thermodynamic stability of lanthanide complexes with amino acids and peptides. Coordination Chemistry Reviews, 249, 567-590. 

Current interest in high temperature chemistry and the closely related thermodynamics of the actinides will provide additional stimuli for determining precise thermodynamic data in cryogenic as well as in higher temperature regions. The utopian era in the chemical thermodynamics of the lanthanides is sufficiently far off to occasion extension of shrewdly devised schemes to other classes of compounds. Use of the semi-empirical schemes already discussed—or theoretically based ones— plus the key compound concept may prove as effective here (desipte magnetic and electronic complications) as it has for hydrocarbon thermodynamics. 

The absence of reliable thermodynamic data for the tetrafluorides has contributed to difficulties in defining the chemistry of the rare earth elements. The fact that only Ce, Pr, and Tb form stable Rp4(s) phases has been established (see section 2.4) however, the thermochemistry of these fluorides has remained uncertain. Insight is provided by the work of Johansson (1978), who has correlated data for lanthanide and actinide oxides and halides and derived energy differences between the trivalent and tetravalent metal ions. The results, which have been used to estimate enthalpies of disproportionation of RF4 phases, agree with preparative observations and the stability order Prp4< TbP4 < CeP4. However, the results also indicate that tetravalent Nd and Dy have sufficient stability to occur in mixed metal systems like those described by Hoppe (1981). 

Piguet C. Chapter 272 — Microscopic thermodynamic descriptors for rationalizing lanthanide complexation processes. In Biinzli J-C, Pecharsky VK, eds. Handbook on the Physics and Chemistry of Rare Earths. Amsterdam, The Netherlands Ekevier 2015 209-271 vol. 47. 

The separation factor is a complex function of the chemistries of the metal ions in the two phases. In a strict thermodynamic interpretation, separation factors are also a function of ion activity coefficients in both organic and aqueous phases. However, since 5[ J/ is a ratio of concentrations of closely similar species, the activity coefficient terms in eq. (3) largely cancel. In most cases, it is probably justified to ignore the potential impact of activity changes on the separation efficiency of the lanthanides, particularly for adjacent lanthanide ions. 

Thermodynamic studies of lanthanide-aminopolycarboxylate complexation have been quite valuable in the understanding of lantiianide coordination chemistry. The same experimental conditions of Grenthe (42,43) were utilized by Choppin,... 

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. 

The most stable oxidation state for all lanthanide elements is the +3 state. This primarily arises as a result of the lack of covalent overlap, which stabilizes low and high oxidation states in the d-block metals by the formation of Ji bonds. While some zero-valent complexes are known, only the +2 and -1-4 oxidation states have an extensive chemistry and even this is restricted to a few of the elements. The reasons for the existence of compounds in the -1-4 and -j-2 oxidations states can be found in an analysis of the thermodynamics of their formation and decomposition reactions. For example, while the formation of all LnF4 and LnX2 is favorable with respect to the elements, there are favorable decomposition routes to Ln for the majority of them. As a result, relatively few are known as stable compounds. Thus L11X4 decomposition to L11X3 and X2 is generally favorable, while most UnX2 are unstable with respect to disproportionation to LnXs and Ln. 

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