Oxalates, solid-state reactions with

Various fluorides may be precipitated from aqueous solution for use as constituent powders in solid state reactions. Co-precipitation offers very elegant access to intimate mixtures, but the actual products are strongly dependent on the fluoride ion activity within the solution but also on the stability constants of the respective metal complexes. Accordingly, not only anhydrous fluorides are obtained, but also hydrated fluorides or hydroxide fluorides, which may be very difficult to convert to pure fluorides. As noted already [3], reactive compounds, e.g. carbonates, acetates, oxalates, hydroxides etc., which quite easily dissolve in acidic HF solutions, are the preferred starting materials for fluoride syntheses. In contrast, many oxides which have been heated to rather high temperature are frequently unreactive and may not dissolve at all. To enhance reactivity but also improve crystallinity of the product, it has proved useful to perform reactions above the boiling point of water in adapting the hydrothermal method, which has already been shown to be useful in the recrystallisation of materials which are more or less insoluble at ambient temperatures and pressures. Up to about 240°C even PTFE vessels may be used. A number of selected examples with respective reaction conditions are listed in Table 3. 

One of the difficulties with the classical solid-state reaction is that mechanical mixing methods are relatively ineffective in bringing the solid reactants in contact with one another. Diffusion lengths, on an atomic scale, are still enormous and the temperatures required may preclude the formation of phases that might be stable at intermediate temperatures. One method, called a precursor method, involves the formation of a mixed-metal salt of a volatile organic oxyanion such as oxalate by wet chemical methods, which result in mixing essentially on the atomic level. The salt is then ignited at relatively low temperatures to form the mixed-metal oxide. The method has been applied successffilly to the preparation of a number of ternary transition metal oxides with the spinel structure. ... 

Many of the perovskite compounds can be prepared by high-temperature solid-state reaction of binary oxides or peroxides. However, some of these tend to be refractory and hence unreactive. Others have a tendency to hydrate or oxidize and are inconvenient to use. Therefore, it is preferable to use carbonates, oxalates, or other easily decomposable compounds, assuming they can be obtained with suitable purity. These materials are usually in the form of fine powders and decompose in the initial stages of the reaction so that faster reaction rates are obtained. In some cases fine powders may be obtained by... 

Fig. 13. Possible structures for (acyloxy)boranes formed from the reaction of 9-BBN with oxalic acid and solid state structure of the complex with 2,2-dimethylmalonic acid 30...

The solid-state reactivity of the carboxylic function was demonstrated with oxalic acid dihydrate (346) and o-phenylenediamine (46a) (Scheme 54). A 100% yield of quinoxalinedione (347) is easily obtained upon cogrinding of the components and heating of the high-melting salt thus formed in a vacuum to 150 °C for 8 h, to 180 °C for 30 min, or to 210-220 °C for 10 min [104]. Compound 347 is ready for further interesting condensation reactions [104]. 

Figure 13.10. Schematic representation of the oxide dissolution processes [exemplified for Fe(III) (hydr)oxides] by acids (H ions), ligands (example oxalate), and reductants (example ascorbate). In each case a surface complex (proton complex, oxalato and ascorbato surface complex) is formed, which influences the bonds of the central Fe ions to O and OH on the surface of the crystalline lattice, in such a way that a slow detachment of a Fe(III) aquo or a ligand complex [in case of reduction an Fe(ll) complex] becomes possible. In each case the original surface structure is reconstituted, so that the dissolution continues (steady-state condition). In the redox reaction with Fe(III), the ascorbate is oxidized to the ascorbate radical A . The principle of proton-promoted and ligand-promoted dissolution is also valid for the dissolution (weathering) of Al-silicate minerals. The structural formulas given are schematic and simplified they should indicate that Fe(III) in the solid phase can be bridged by O and OH.

Other decompositions, which had previously been accepted as simple reactions proceeding in the solid state, have subsequently been shown to be more complicated than was discerned from overall kinetic data. The thermal breakdown of potassium permanganate exhibits almost symmetrical sigmoid curves, now regarded (39) as proceeding with the intermediate formation of K3(Mn04)2 by at least two, possibly consecutive, reactions. Dehydration of calcium oxalate monohydrate proceeds (75) with the loss of H20 molecules from two different types of site by two concurrent reactions that proceed at slightly different rates. 

Reactions of materials in the solid state are strongly influenced by an enormous range of variables, and a complete treatment of this vast subject is beyond the scope of this book or, in fact, any single volume. One factor that becomes apparent immediately when dealing with soHd state reactions is that the rate can generally not be expressed in terms of concentrations. We can illustrate this by means of the following example. The first step in the decomposition of metal oxalates when they are heated normally leads to the loss of carbon monoxide and the formation of a carbonate. In the case of NiC204, the process can be shown as... 

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