Lanthanides thermodynamic parameters

In contrast to the situation observed in the trivalent lanthanide and actinide sulfates, the enthalpies and entropies of complexation for the 1 1 complexes are not constant across this series of tetravalent actinide sulfates. In order to compare these results, the thermodynamic parameters for the reaction between the tetravalent actinide ions and HSOIJ were corrected for the ionization of HSOi as was done above in the discussion of the trivalent complexes. The corrected results are tabulated in Table V. The enthalpies are found to vary from +9.8 to+41.7 kj/m and the entropies from +101 to +213 J/m°K. Both the enthalpy and entropy increase from ll1 "1" to Pu1 with the ThSOfj parameters being similar to those of NpS0 +. Complex stability is derived from a very favorable entropy contribution implying (not surprisingly) that these complexes are inner sphere in nature. 

Table 2 Thermodynamic Parameters of Hydrated Lanthanide Ions191 192...

Choppin [24] examined some aspects of lanthanide-organic ligand interaction in aqueous solutions. An interpretation of thermodynamic parameters (AG, AH and AS) of complexation have been given in terms of hydration, inner versus outer sphere character, stability vs. chelate ring size and ligand charge polarization. 

The thermodynamic parameters (i.e.) the enthalpy and entropy values showed the formation of inner-sphere chloro complexes in the case of all the lanthanides. The enthalpies for the formation of monobromo complexes of lanthanides are also positive but smaller in magnitude than the corresponding chloro complexes. The complex formation enthalpies follow the sequence A//°(C1) > A//°(Br) > A//°(I) which is unusual for hard metal(III) ions. 

The thermodynamic parameters of the thiocyanato complexes are similar to those of lanthanide bromo complexes and the latter are outer-sphere complexes in DMF. Contrary to this 89Y NMR spectra suggest an inner-sphere Y(III)-NCS complex. 

Infrared evidence shows N-bonding to lanthanides. The near constancy of the thermodynamic parameters for thiocyanato complexes is due to small N-end and lack of geometry change along the lanthanide series. In alcoholic solutions, inner-sphere thiocyanato complex formation has been observed [127] with cerium (III). 

With ethylenediamine complexes of the formula Ln(en)3X3 and Ln(en)4X3, where X = C1 , Br , NO, CIOJ have been characterized. IR data indicate that the tris and tetrakis complexes of the fighter lanthanides La-Sm, contain both ionic and coordinated nitrate groups. By contrast tetrakis complexes of heavier lanthanides, Eu-Yb contain ionic nitrate. This is possibly due to steric factors resulting from decreasing cationic radius that force the nitrate out of the coordination sphere of the lanthanides. A coordination number of 8 for tris complexes and a number of 9 for fighter lanthanide tetrakis complexes appears reasonable [234]. The thermodynamic parameters obtained show enthalpy stabilization for... 

Since tropolones and 3-hydroxy-4-pyrones are taken in this chapter to be enols, we now cite their binding as enolato ligands to lanthanum and all the other trivalent lanthanides (save the radioactive promethium) " " . Likewise, we note such studies for complexes with enolato ligands derived from 3-acetyl-4-hydroxycoumarin, dehydroacetic acid and their oximes, and with the aromatic enediolates, squarate and croconate. Periodic trends in thermodynamic parameters were reported and analyzed in these studies. 

Solvent extraction, potentiometry, and calorimetry have been used to determine the thermodynamic parameters of the formation of the monofluoride complex of the trivalent lanthanide ions at 25°C. and an ionic strength of IM (NaClOj ). The enthalpies were all endothermic, ranging from 4.0 to 9.5 Kcal./mole consequently, the large, positive entropies, ranging from 25 to 48 cal./°C./mole, explain the high stability constants. This large entropy results from the decrease in overall water structure when the fluoride ion is complexed. The difference in the enthalpies of formation of LnF and LnAc " can possibly be explained by a difference in covalence for Ln-F and Ln-O bonds. 

The closest redox-stable analogue of Ce(IV) is thorium(IV), for which a large data base of thermodynamic parameters is available for the carboxylic add complexes (Martell and Smith 1977). Using the ionic radii of Shannon (1976) and recalling that the stability of lanthanide and actinide complexes is derived almost exclusively from electrostatics, we can estimate that a 16% increase in the log of the stability quotients for thorium (since AG oc Z /r oc log should provide a reasonable estimate for the corresponding complexes of cerium(IV) [rce(CN = 8) = 0.97 A, r iCN = 10) = 1.13 A, (l/rce)/(l/ xh) = 116, CN = coordination number]. 

Fig. 13. Thermodynamic parameters for 1 1 and 1 2 complexes of lanthanides with hiba and isobutyric add AG (squares), AH (circles), AS (triangles). Solid symbols are 1 1 complexes, open symbols for step-wise...

Goedken, and Gritmon(J3) in 1977 to investigate the thermodynamic parameters of a variety of aminopolycarboxylate ligands across the lanthanide series. Variations in the enthalpy and entropy curves vs. Z were attributed to different degrees and patterns of dehydration as well as the increasing polydentate nature of the complexation. The linear relationship between log 6 and ZpKa of... 

Bulk data of solvation of tetravalent cations supports octa-coordination in the first hydration sphere whereas the actinyl(VI) cations seem to favor hexa-coordination in the plane perpendicular to the axial actinyl group. The trends in the thermodynamic parameters of hydration across the actinide series are essentially similar to those observed for the lanthanide ions and the small differences can be attributed to relativistic effects in the actinides which causes changes in the relative energies of the s, p, d, and f orbitals. 

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. 

Franciosi et al. 1984) and Tm is trivalent (Fujimori et al. 1986). This observation alone defines a relatively narrow range for the difference in adsorption energies for the divalent and trivalent states of the lanthanide atoms. In the future, accurate experimental recordings of the energy positions of 4f levels by photoelectron spectroscopy and BIS for this type of systems should be most valuable. Thereby the quantification of important thermodynamical parameters for the interfacial interaction could be facilitated. 

Gas-phase chemistry studies of atomic and molecular rare-earth and actinide ions have a deep-rooted history of more than three decades. In gas phase, physical and chemical properties of elementary and molecular species can be studied in absence of external perturbations. Due to the relative simplicity of gas-phase systems compared to condensed-phase systems, solutions or solids, it is possible to probe in detail the relationships between electronic structure, reactivity, and energetics. Most of this research involves the use of a variety of mass spectrometry techniques, which allows one exerting precise control over reactants and products. Many new rare earth and actinide molecular and cluster species have been identified that have expanded knowledge of the basic chemistry of these elements and provided clues for understanding condensed-phase processes. Key thermodynamic parameters have been obtained for numerous atomic and molecular ions. Such fundamental physicochemical studies have provided opportunities for the refinement and validation of computational methods as applied to the particularly challenging lanthanide and actinide elements. Among other applications, the roles of... 

To reliably select separate thermodynamic parameters for the compoimds under consideration, we must improve the suitable calculation procedures and/or criteria that allow the validity of the available experimental data to be estimated. To this end, we have selected lanthanide trifluorides as example. They are much less sensitive to hydrolysis although they have noticeable corroding action on container materials at high temperatures. In view of this, we have chosen the method of heat capacity calculations over a wide temperature range, up to the melting point, based on an analysis of the experimental low-temperature heat capacities and high-temperature enthalpy increments. This method is described below. Its application to lanthanide di- and trichlorides will be presented later on. 

The calculation scheme for the enthalpies of formation of lanthanide dihalides proposed by Kim and Oishi (1979) is based on the assumption that the formation of these compounds, except for europium and ytterbium, is accompanied by an electronic transition 4f 5d 6s 4f + 5d°6s in the lanthanide atom. This results in the observation of an irregularity in the variation of the thermodynamic parameters, including the enthalpies of formation, as a function of the lanthanide atomic nmnber. 

From the latter equation the calculated boiling point of Cm is 3110°C. The derived heat of fusion, entropy of fusion, and average second-law entropy are 13.85 kJ mol , 9.16 JK mol , and 106.7 3.0 J K mol , respectively. Low-temperature condensed-phase thermodynamic parameters await the availability of long-lived isotopes. For excellent discussions of thermodynamic, electronic, and magnetic effects in curium and other actinide and lanthanide metals, the reader is referred to recent articles by Ward and Hill [34]. 

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