Complex halides thermodynamic properties

A critical assessment of the chemical thermodynamic properties of the actinides and their compounds is presently being prepared by an international team of scientists under the auspices of the International Atomic Energy Agency (Vienna). As a result of this effort, four publications (1, 2, 3, 5) have already become available and a further ten 6-T4), including the halides (8) and aqueous complexes with Tnorganic ligands (12),... 

Geoffroy, George, L., Photochemistry of Transition Metal Hydride Complexes George, J. W., Halides and Oxyhalides of the Elements of Groups Vb and VIb George, Philip and McClure, Donald S., The Effect of Inner Orbital Splitting on the Thermodynamic Properties of Transition Metal Compounds, and... 

AH of reaction (17) is an estimate since PuCl4(s) has not been synthesized AfH° [PuCl4(s)] = — 964 kJ is an estimate (Fuger et al. 1983). The dramatic difference in these enthalpies [Pu(IV) is more stable than Pu(III) in this complex chloride by -95-( —4) = - 91 kJ or almost 1.0 V, in comparison with the binary chloride] shows how acid-base effects influence oxidation-reduction properties. As noted above, there are few known lanthanide (IV) complex halides and no thermodynamic data on even these few halides, so no quantitative comparison can be made. Nevertheless it does show how complexation effects by basic complexants make high f-element oxidation states attainable. Perhaps the most dramatic evidence of the enhancement of high oxidation state by a basic fluoride is the existence of the Nd(IV) and Dy (IV) compounds such as Cs2(Cs, Rb, K) (Nd, Dy)Cl2 these AjRF, double fluorides are the only known Nd (IV) and Dy(IV) compounds. 

The insufficiency of the literature data determined the sequence with which the thermodynamic properties of lanthanide halides were considered in our work. We first described RF3 (Chervonnyi and Chervoimaya, 2007a), next RCI3 (Chervoimyi and Chervonnaya, 2007b), and, finally, RCI2 (Chervoimyi and Chervonnaya, 2008a). This description was preceded by complex calculations of thermodynamic equilibria in the case of samarium, europium, and ytterbium chlorides (Chervonnyi and Chervonnaya, 2004a,e,f, 2005b). 

Needs of the industrial technologies called for extensive studies and measurement of the physicochemical properties of halides and oxohalides their thermodynamic characteristics, phase diagrams, reactions, complexing in gases and so forth. After two decades of growth, the intensity of these works waned today, such studies are scarce, though the properties of a number of compounds are still known with low accuracy. 

Selected physical properties of the tetrahalides of C and Si are listed in Table 13.3. The carbon tetrahalides differ markedly from those of the later group 14 elements they are inert towards water and dilute alkali and do not form complexes with metal halides. The distinction has been attributed to the absence of d orbitals in the valence shell of a C atom look back at the electronic versus steric debate, outlined in Section 13.3. However, one must be cautious. In the case of CX4 being inert towards attack by water, the lack of C d orbitals presupposes that the reaction would proceed through a 5-coordinate intermediate (i.e. as is proposed for hydrolysis of silicon halides). Of course, it is impossible to establish the mechanism of a reaction that does not occur Certainly, CF4 and CCI4 are thermodynamically unstable with respect to hydrolysis compare the value of AjG° for equation 13.35 with that of —290 kJ mol for the hydrolysis of SiCl4. 

The systematics of high-temperature metal halide vapors and vapor complexes have earlier been summarized in a number of review articles or monographs. Worth mentioning are the books by Schafer (1964), Margrave (1967) and Hastie (1975) and a number of proceedings volumes, such as those edited by Hildenbrand and Cubicciotti (1978) and Hastie (1979). Schafer (1976, 1983) reviewed the thermodynamics of metal chloride vapor complexes. Papatheodorou (1982) and Brooker and Papatheodorou (1983) gave an extensive account on the electronic, vibrational, thermodynamic and structural properties of transition and rare-earth metal halide vapor complexes. Finally, Hilpert (1989, 1990)... 

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