Halide thermodynamics considerations

The most obvious route to a binary compound ExE y is the direct interaction between the elemental substances E and E. This is often precluded by thermodynamic considerations however, a great many compounds that are unstable with respect to the elemental substances are kinetically stable and can be prepared by other means. Even if the reaction between the elemental substances is possible, both from thermodynamic and kinetic viewpoints, an indirect synthesis may be preferable for practical reasons. We shall illustrate these principles by looking at some examples of synthetic routes to binary hydrides and halides. 

Exchange energy associated with nearly half-filled and filled configurations of samarium(ll) and (huliumdl) is not thought to contribute much to the stability of halide salts. See Johnson, D. A. J. Chem. Educ. 1980. 57, 475-477 for thermodynamic considerations. 

The use of gaseous metallic halides for the production of metallic coatings involves further thermodynamic considerations. In addition to direct cementation, three types of surface reaction, resulting in metal deposition, can occur [58] ... 

The most widely used general method of producing the rare metals in a pure condition is by reduction of the oxides or halides. Potentially suitable reducing agents can be selected from thermodynamical considerations. 

Although there has been some recent interest in the catalytic fluorination of CCI4, notably over chromium(III) fluoride derived from thermal decomposition of (NH4)3CrF6 [42], most attention has been paid to dismutations within the HCFC—CHCl3 F series. Reactions on several metal oxides and halides have been studied by Kemnitz and coworkers and evidence for dismutation behaviour adduced from thermodynamic considerations and... 

For all three halates (in the absence of disproportionation) the preferred mode of decomposition depends, again, on both thermodynamic and kinetic considerations. Oxide formation tends to be favoured by the presence of a strongly polarizing cation (e.g. magnesium, transition-metal and lanthanide halates), whereas halide formation is observed for alkali-metal, alkaline- earth and silver halates. 

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]. 

Lever has successfully predicted Mn"/ potentials of 24 Mn-carbonyl complexes containing halide, pseudohalide, isonitrile, and phosphine co-ligands, with additivity parameters derived from the potentials of Ru "/" couples [39]. An important consideration for heteroleptic complexes is the influence of isomerism on redox thermodynamics. For Mn(CO) (CNR)6- complexes, with n = 2 or 3, the Mn"/ potentials for cis/trans and fac/mer pairs differ by as much as 0.2 V [40]. The effect arises from the different a-donor and 7r-acceptor abilities of carbonyl (CO) and isocyanide and their influence on the energy of the highest energy occupied molecular orbital (HOMO). 

In an extensive, comparative study of a range of glycosyl halide derivatives in their thermodynamically more-stable forms, it was uniformly found that the conformation adopted in solution is the one having the halogen group axial, suggesting that the anomeric effect must be of considerable magnitude (29, 30). 

Of course, the traditional problem of the lack of precise knowledge of the heats of solvation for the passage of these ions into solution, makes the above criteria of stability less valuable to the condensed-phase chemist. A major breakthrough in this classical impasse has been achieved by Arnett and coworkers " who have recently carried out calorimetric measurements leading to reliable values of the enthalpy of ionisation of various alkyl, cycloalkyl and aiyl halides in solution. These determinations owe their validity to the use of superacid conditions and the NMR verification that the ions expected were in fact formed in those media without Ihe occurrence of secondary reactions. One of the most important conclusion of these studies is that on the whole the relative stabUities of carbenium ions are the same in the gas pha% and in the solvents used, i.e., electrostatic solvation effects do not alter the order of stability. The importance of this new experimental approach is quite obvious and one can except in the near firture considerable advances in the field of the thermodynamics of reactive carbenium ions in solution through the attmnment of a precise knowledge of AG° values for their formation in various media. 

Hence, it can be concluded that peroxide ions are thermodynamically unstable in high-temperature melts of alkali metal halides. However, in the presence of 02 in the atmosphere over the halide melts they may exist in sufficient concentrations, even at temperatures considerably exceeding that of decomposition of pure alkali metal peroxides. 

All the routines described for the determination of the thermodynamic (concentration) parameters in metal oxide solutions include some indirectly obtained values. For example, the equilibrium concentration of metal cations is calculated proceeding from the quantity of the oxide-ion donor consumed for titration (precipitation). Direct determination of the concentration of metal cations in the melt (if it is possible) allows one to obtain more correctly the obtained solubility product values. Our paper [332] reports a method for correction of the solubility product values for oxides on the basis of the potentiometric titration data. The modification of the standard routine consists of the simultaneous use of two indicator electrodes, one of which is the membrane oxygen electrode and the other is a metal electrode, reversible to the cations the oxide consists of. This routine was used to estimate the solubility products of copper(I) and nickel(II) oxides in the molten KCl-NaCl equimolar mixture at 700 °C. Investigation of Cu20 by the proposed method is of considerable importance since, as will be shown further, the process of dissociation/dissolution of copper(I) oxide in molten alkali-metal halides differs from the generally accepted one which was considered, e.g. in Ref. [119]. 

This approach has been used to study the mechanism of a bond-breaking reaction following electron transfer (a dissociative electron transfer). Consider, for example, the case where species Z is an aryl halide, ArX, that becomes reduced by the electrogenerated re-ductant to yield the ultimate products Ar and X . This result can occur either by a concerted path, where bond cleavage occurs simultaneously with electron transfer, or by a stepwise path, where the radical anion, ArX", is an intermediate. Investigations of such reactions have been carried out by redox catalysis, and theoretical analysis of the structural and thermodynamic factors that affect the reaction path have been described (14, 41, 42). Similar considerations apply to oxidation reactions, such as of C20l to form two molecules of CO2. 

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