Hydroxo salts

This was rejected by Werner,2 as it did not agree with the properties of the substances. Thus, the salts do not yield aquo-salts on heating with dilute mineral acids, therefore the hydroxyl group cannot be directly united to cobalt as in the hydroxo-salts. The chloride of the series cannot have chlorine in the complex, otherwise the salt could not... 

These hydroxo-salts are all sulphur-yellow crystalline substances. The acid residues are hydrolysable and hence outside the co-ordination complex, and the aqueous solutions, unlike the hydroxo-salts of chromium-and cobalt-ammines, are neutral to litmus, a fact which Werner suggests is due to the smaller tendency of the hydroxo-radicle attached to ruthenium to combine with hydrogen ions. This tendency is much less than in the case of the ammines of cobalt and chromium, but that it still exists is indicated by the increased solubility of these hydroxo-compounds in water acidified with mineral acids, and from such solutions aquo-nitroso-tetrammino-ruthenium salts are obtained thus ... 

Alkoxides and aryloxides demonstrate similar chemistry to that of hydroxides in that it is possible to prepare mixed metal or double metal derivatives similar to hydroxo salts such as Na2[Sn(OH)6]. The formation of mixed metal alkoxides, e.g. Na2[Zr(OEt)6] and Ln[Al(OPri)4]3, is typically a result of the electron deficiency (Lewis acidity) of the metal centers in units of the type M(OR)x or M(OAr)JC (x < 5). This then leads either to oligomerization via alkoxide bridges or, in the presence of other alkoxides, to the formation of mixed metal compounds. 

Table 4 Metal Ions Which Fonn Hydroxo Salts ...

Hydroxo salt is the term used for a group of complexes where the central atom of the complex anion is a metal to which hydroxyl ions are bound as ligands. The niunber of these ions depends on the normal coordination number of the metal. The cation of a hydroxo salt is usually an alkali metal, particularly sodium, or the alkaline earth metals barium, strontium and, in some cases, calciiun. Heavy-metal salts can be prepared from a few hydroxo anions via a double decomposition reaction. 

Hydroxo salts correspond closely to the well-known halo salts in their formula type and structure. Accordingly, mixed halo-hydroxo salts of a number of metals can be prepared. At elevated temperature, hydroxo salts can be converted into oxo salts, provided it is feasible to prepare the latter from a metal hydroxide (the central atom) and an alkali or alkaline earth metal hydroxide. In numerous systems, however, such oxometalates can be obtained only from oxides. In some cases, oxohydroxo salts are also formed as Intermediates. 

The reagents are strong bases and poorly soluble metal hydroxides. In the above equations, the metal hydroxides function as acids, in agreement with the modern theory of their amphoteric behavior. As far as is presently known, the following metals (arranged in order of increasing valence) form hydroxo salts ... 

The equilibrium distribution is of controlling importance in the preparation of crystalline alkali hydroxometalates. Most hydroxo salts, alkali salts in particular, rapidly decompose into their components in the presence of H3O or dilute alkali hydroxide. Only the hexahydroxo salts of Sn (IV), Pt (IV) and Sb (V) dissolve in HgO at room temperature without decomposition, whereas the other alkali hydroxometalates are stable only when they constitute the solid phase in the presence of (usually very concentrated) alkali hydroxide solution. The alkaline earth hydroxometalates are relatively sparingly soluble and hence are stable in dilute hydroxide the compound Ca[Zn(OH)3]3 2 H3O is stable even in water. The equilibrium shifts toward the reagents with temperature. 

The experimental fact that the sodium hydroxide concentration at the solubility maximum is usually over 30% indicates that highly concentrated (usually 45-50%) NaOH is required for the preparation of most alkali hydroxo salts. This is also necessary to ensure good yields. However, most alkaline earth hydroxometalates can be prepared from more dilute NaOH solutions. 

Several types of hydroxo salts of some di- and trivalent metals can be prepared just as in the case of halo salts. These differ in the number of coordinated OH" ions. The number of OH" ligands depends on the concentration and temperature of the alkali hydroxide. 

In some cases the fact that the hydroxometalates are chemical complexes is indicated by the color of the salts and of their solutions. The proof of structure is based on their thermal dehydration curve, their ability to form mixed halo-hydroxo salts, data on isomorphic relations, and some powder pattern studies. 

Thermal decomposition of [MnfCjO yH O jK in solution. On dissolution in water a partial change takes place in hydroxo salt (1), according to... 

Thermal decomposition of [Mn(C O UOHyOH )]K in solution. By dissolving the diaqua complex in KOH (pH 10) equilibrium 1 is shifted to the right, a brown solution of hydroxo salt is formed accompanied with an increase of the extinction while the pH decreases to 4. Then the thermal decomposition of the hydroxo complex takes place, accompanied with a rise of the pH. The decomposition mechanism can be represented by the system of reactions ... 

The hydroxo salt concentration will thus evolve according to... 

Solution containing a mixture of [MnfC.OjUH.OljK and [Mn(CyO,)y(OHXOH )3K. By the diaqua complex, a mixture of diaquo and hydroxo salts is produced, according to l. The two ions decompose in a reaction of the first order. The rate constants of the decomposition of the diaqua complex ate approximately twice larger. The experimental data show that on this interval of temperature the equilibrium of reaction l is noticeably displaced to the right with the formation of hydroxo-complex in a predominant proportion. 

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