Double complex salts

Related to these enzyme -catalysed reactions are electroreductions of acetophenone in the presence of chiral crown ethers. The low optical yields (<3%) are attributed to association of the prochiral substrate and the chiral crown ether salt complex in the electrochemical double layer (Horner and Brich, 1978). 

Coprecipitation of the metals is usually achieved from an aqueous solution of nitrates upon addition of anions such as carbonates, citrates, or oxalates (10)(24-27). First reports in this field have underlined the necessity to neutralize the pH of the solution in order to obtain complete precipitation of barium or strontium. Also, oxalate or citrate ligands may bind to two different cations. This should allow a better mixing at a microscopic level. However, care should be taken since some cations such as Y or La may precipitate as double salt complexes with alkaline ions that have been added to the solution as hydroxides in order to control the pH (24). 

The co-ordination theory lias also been applied to many complex salts and double salts other than ammines. For example, potassium ferrieyanide may be represented by the formula... 

Chromates are usually yellow or red in colour, and, except those of ammonium, the alkali metals, calcium, strontium, and magnesium, are practically insoluble in w ater. They are obtained by oxidation of chromites, by fusion of chromium sesquioxide with the appropriate base in presence of air or of an oxidising agent by oxidation of chromium salts in solution or by double decomposition. Normal, di-, and tri-clrromates, etc., are derived from one and the same acid oxide KaCrOj behaves like an alkali torvards CrOg, since it is quantitatively converted into dichromate. A large number of complex double chromates are known. 

Hydrocyanic acid is most easily prepared from its potassium salt, K(CN), which is obtained principally by the decomposition of the complex double cyanides of iron as we shall soon consider. The acid is also obtained by the hydrolysis of certain glucosides, e.g., amygdalin, in bitter almonds. It is prepared synthetically by reactions to be discussed presently in connection with the constitution of it and its salts. It is a colorless liquid with a characteristic odor and burns with a violet flame. It boils at 26.1 and solidifies to crystals which melt at —14°. It is an extremely strong poison the best antidotes being chlorine and hydrogen dioxide. It is readily absorbed by metallic nickel which is thus used in gas masks for this purpose. It is stable in dry air but in presence of water is readily hydrolyzed yielding ammonia and formic acid as the chief products. 

The use of tin involves a number of difficulties. The chlorides of tin produced in the presence of an excess of hydrochloric acid give chlorostannic acid, which combines with aniline and some other arylamines to form complex double salts as shown in equation (4). In order to decompose these salts alkali is used. A great excess must be employed in order to change the stannic hydroxide, which i,s formed, to soluble sodium stannate ... 

Werner discarded Kekule s distinction between valence compounds, which are eminently explainable using classical valence theory, and molecular compounds, which are not. Werner proposed a new approach in which the configurations of some compounds— metal-ammines (now sometimes called Werner complexes ), double salts, and metal salt hydrates— were logical consequences of their coordination numbers (a new concept) and two types of valence, primary and secondary. For compounds having coordination number six he postulated an octahedral configuration for those having coordination number four he proposed a square planar or tetrahedral configuration. 

There are two distinct types of coordination compounds separated from one another by their reactivity, which is due to the nature of the bonding from the metal to the ligand. Complex compounds are bound by coordinate covalent bonds described by valence bond theory (see Chapter 6). Addition salts or double salts, however, are bound according to electrostatic interactions, or ionic bonds. (Turn to Chapter 8 for details on ionic bonds and salts.)... 

A double salt compound is stable in a solid state, but dissociates, or dissolves, into ions in an aqueous environment. They are called double salts because they are usually composed of two metal elements. (See Chapter 8 for details on the solubility of salts.) Complex compounds are also stable in solid state, but when added to an aqueous solution they maintain their structure rather than dissolving into ions. The addition salts or double salts also typically have lower coordination numbers than the complex compounds. 

Solubility depends on using the appropriate ligand. The ligand can alter the solubility and, therefore, also the suitability for certain applications. There are many organometallic double salt complexes used in industry. In such a complex, there are two metal atoms surrounded by several ligands that are associated with the metals. The most common ligand is water. In aqueous solution, the ions are hydrated (see Chapter 8). Water is an example of a neutral ligand. 

Uranium tetrafluoride forms stable coordination complex fluorides with ammonium fluoride and many alkali and alkaU-earth fluorides like NH4UF5 or Na2UFg and several other fluoride compounds. The melting point of the complex double salts formed in the NaF-Up4 system is much lower than that of the components (Galkin 1966). 

Thus section B is simplified because double salts, hydrates, and solutions are only discussed for the first class of compounds. It is easy to conclude that the experimental basis for the first class of compounds done supports the important relationships between complexes, double salts, hydrates, and the physical chemistry of solutions. Moreover, those topics common to both sections are allotted very different amounts of space in section A, 16.5 pages are devoted to the formation of compounds (A I), the replacement of ammonia molecules by water molecules (A III) is allotted 17.5 pages, while the theoretical discussion (A VI) occupies five pages in section B the experimental topics (BI and BII) take up six pages, whereas the theoretical discussion extends to eleven pages. It is fair to conclude that the first subsection is the actual foundation of the whole argument. 

The addition of excess sodium oxalate to solutions of copper or cobalt salts likewise produces complex double oxalates. Consequently the presence of free acid can then be detected by means of indicators in these cases also. 

---