Trivalent thermodynamic values

Representative heats of formation predicted by the ECP/BAC-MP4 method are given in Table 1 (the complete published [88] thermodynamic data set used in the analyses below is available online [91]). Data are shown for a range of compoimds, including tetravalent, trivalent, and divalent coordination at tin. Values for the reference compounds SnCU, SnH4, and Sn(CH3)4 are also given. Finally, heats of formation for atoms and groups needed to calculate reaction enthalpies are given. These results are used in the analysis below to identify potential reaction pathways for MBTC and its decomposition products. 

Thermodynamic data, and especially AS values, are generally unreliable or lacking for important phosphate complexes. Until such AS data is measured, it can be estimated with fair accuracy using the Fuoss equation for monovalent and divalent-bonded complexes and the electrostatic model when trivalent and quadrivalent addends are associated. Unfortunately, published AG and AS data on HS , s2 , and Se and Te aquo-complexes are suspect or largely lacking (Barnes, H. L., Pennsylvania State University, personal communication, 1978). Both the stoichiometry and stability of such complexes remains in doubt. Once a few such data have been accurately measured, plots with EN (10) or Q (44) as a variable, or using hard and soft acid and base concepts (3, 40) should permit the useful estimation of many as yet un-... 

The thermodynamic data for complexation of trivalent lanthanide and actinide cations with halate and haloacetate anions are reported. These data are analyzed for estimates of the relative amounts of inner (contact) and outer (solvent separated) sphere complexation. The halate data reflected increasing inner sphere character as the halic acid pKa increased. Use of a Born-type equation with the haloacetic acid pKa values allowed estimation of the effective charge of the carboxylate group. These values were, in turn, used to calculate the inner sphere stability constants with the M(III) ions. This analysis indicates increasing the inner sphere complexation with increasing pKa but relatively constant outer sphere complexation. 

Tables 9-11 list the predicted thermodynamic functions for the hydration of divalent, trivalent and tetravalent lanthanides as calculated by Bratsch and Lagowski (1985b). Values for yttrium hydration are also included when available. The formation values refer to the reaction Ln, Ln"a while the hydration values relate to the use of eqs. (28)-(30). The standard state ionic entropies given in table 10 are corrected for...

The thermochemistry of rare-earth trifluorides was summarized in Gmelin Hand-buch (1976) and the thermochemistry of rare-earth tribromides and triiodides was summarized in Gmelin Handbuch (1978). The thermochemistry of trivalent rare-earth trichlorides was critically assessed by Morss (1976). Enthalpies of formation of most of the lanthanide tribromides were determined by Hurtgen et al. (1980). Thermodynamic properties for europium halides were assessed by Rard (1985). Only enthalpies of formation of Sc, Y, Dy and Tm triiodides have been redetermined since the classical work of Hohmann and Bommer (Morss and Spence 1992). A recent set of literature values of enthalpies of formation of rare-earth solid and gaseous trihalides has been published, accompanied by Born-Haber cycle estimated values for all trihalides (Struck and Baglio 1992). 

No experimental data for the europium-scandium phase diagram were found. However, Miedema (1976), using Gsehneidneris (1969) value for the energy difference between divalent and trivalent europium (96kJ/g-at) found his method would account for the known information on the valence state of europium in intermetallic compounds. The application of his thermodynamic calculations to the europium-scandium alloy system predicted that no stable compounds exist in this system. 

The average coordination numbers in the nitrate solutions are somewhat larger. This may be explained by the occurrence of smaller steric effects due to the small size of nitrate. Vibrational data do not evidence large inequivalences between the coordinated DMSO molecules, so that the variations in coordination numbers may be interpreted as follows. When excess DMSO is added, the inner coordination sphere of the R(III) ions expands to accept an additional donor molecule, resulting in a slightly weaker R(III)-DMSO interaction. The thermodynamic stabilities of the species [R(DMSO)jt] and [R(DMSO)x+i] " are close, as demonstrated by the small values calculated for log. x+i (table 13). The electronic density on the trivalent ions increases with Z, therefore there are situations in which the expansion of the inner coordination sphere leads to species... 

Thermodynamic data for the complexation of trivalent lanthanides by hexa-cyanoferrate(III) have been interpreted as supporting a model of outer-sphere ion-pairing of RFe(CN)j (Stampfli and Choppin, 1972). The stability constants for the interaction of Fe(CN)g and Co(CN)j ligands with selected R ions have been determined and for La at zero ionic strength the log values... 

Standard oxidation potentials referred to the normal hydrogen electrode (E°) for the divalent and trivalent lanthanide aquo ions are given in table 24.8 based primarily on the selected experimental results compiled by Chariot et al. (1971) and the systematic correlations summarized by Nugent (1975). Only Eu and Yb can persist for times of the order of minutes to hours in dilute acid solution, and can be readily produced by reduction of the trivalent ionic species (Laitinen and Taebel, 1941 Laitinen, 1942 Christensen et al., 1973). The value of E° = -0.43 V for the Eu(II-III) couple quoted in many previous compilations was obtained using both potentiometric and polarographic techniques. However, in a reevaluation of the solution thermodynamic properties of europium, Morss and Haug (1973) recommended the value E° = —0.35 V. 

---