Polynuclear hydroxo complexes

It is well known that hydrolyzed polyvalent metal ions are more efficient than unhydrolyzed ions in the destabilization of colloidal dispersions. Monomeric hydrolysis species undergo condensation reactions under certain conditions, which lead to the formation of multi- or polynuclear hydroxo complexes. These reactions take place especially in solutions that are oversaturated with respect to the solubility limit of the metal hydroxide. The observed multimeric hydroxo complexes or isopolycations are assumed to be soluble kinetic intermediates in the transition that oversaturated solutions undergo in the course of precipitation of hydrous metal oxides. Previous work by Matijevic, Janauer, and Kerker (7) Fuerstenau, Somasundaran, and Fuerstenau (I) and O Melia and Stumm (12) has shown that isopolycations adsorb at interfaces. Furthermore, it has been observed that species, adsorbed at the surface, destabilize colloidal suspensions at much lower concentrations than ions that are not specifically adsorbed. Ottewill and Watanabe (13) and Somasundaran, Healy, and Fuerstenau (16) have shown that the theory of the diffuse double layer explains the destabilization of dispersions by small concentrations of surfactant ions that have a charge opposite to... 

It has been suggested (61) that the hydroxo-complex ions exclusively are the PDI when they are present and that highly charged, polynuclear hydroxo complexes play the major potential- or charge-determining role. Empirically, the IEP(aq), where ... 

The situation is further complicated by frequent observation of IEP(s)s which do not approximate the appropriate IEP(aq). The variability of the IEP(s) probably arises partly from the presence of multivalent, polynuclear hydroxo complexes. The compositions and concentrations of these species depend strongly on the over-all composition and history of the solution (42, 44). If the influence of such complexes is inordinate, the difficulty of obtaining equilibrium among them will be manifested in apparent variability of the IEP(s). 

The establishment of hydrolysis equilibria is usually very fast, as long as the hydrolysis species are simple. Polynuclear complexes are often formed rather slowly. Many of these polynuclear hydroxo complexes are kinetic intermediates in the slow transition from free metal ions to solid precipitates and are tlius thermodynamically unstable. Some metal-ion solutions age, that is, tliey change their composition over periods of weeks because of slow structural transformations of the isopoly ions. Such nonequilibrium conditions can frequently be recognized if the properties of metal-ion solutions (electrode potentials, spectra, conductivity, light scattering, coagulation effects, sedimentation rates, etc.) depend on the history of the solution preparation. 

Mononuclear Wall If hydrolysis leads to mononuclear and polynuclear hydroxo complexes, it can be shown that mononuclear species prevail beyond a certain dilution. If we consider, for example, the dimerization of CuOH" ... 

Electrostatic models such as the one presented in [94PLY/GRE] have been used to estimate the temperature dependence of formation constants and entropies of hydrolysis reactions of polynuclear hydroxo complexes, such as Ni4(OH)4. Such semi-theoretical treatments are beyond the scope of this review. 

Davydov, Yu. P., Toropov, I. G., Hydrolysis of thorium(4+) cations with formation of polynuclear hydroxo complexes in solutions, Dokl. Akad. Nauk Belarusi, 36, (1992), 229-233, in Russian. Cited on page 593. 

These phenomena find expression in the presence of supplementary equilibria that actually govern them and that interfere with those corresponding to the complex under interest. Constants Kjj (or p ), together with those already handled with mononuclear complexes, permit us to calculate the concentrations of the different species, including those of the polynuclear hydroxo complexes. 

Several studies of the circular dichroism (CD) spectra of aqueous solutions of the lanthanides in the presence of optically active ligands have been made (Katzin, 1970 Misumi et al., 1974 Prados et al., 1974). The CD spectra are highly structured in the region of the intra-4f transitions and are very sensitive to change in the pH of the solution particularly at values for which polynuclear hydroxo complexes could be formed. 

Davydov, Y.P. and Efremenkov, V.M. (1975) Investigation of the hydrolytic properties of tetravalent uranium. II. Conditions of formation of mononuclear and polynuclear hydroxo complexes of U(IV) in solution. Radiokhimiya, 17, 160-164. 

Burkov, KA. and Ivanova, L.V. (1966) Formation of polynuclear hydroxo complexes of nickel in sodium nitrate medium. Vestn. Leningr. Univ., Ser. 4 Fiz. Khim, 16, 120-129. 

Figure l.4 The structural variety of trinuclear ferric hydroxo complexes, Fe3(OH)f i, that could form from Fe2(OH)24+ at low pH. Iron atoms are indicated as full circles, OH as open circles. The arrow indicates a transition from dinuclears to trinuclears (p = 2 to p = 3), which is realistic as an early step in the growth of polynuclears. (From Crichton, 1991.)... 

Since A1(N03)3 or its salt hydrates dissociates to A13+ and NOs ions in the aqueous solution, its reactions in solutions are those of Al. It is partially hydrolyzed, producing H30" and thus accounting for the acidity of its solution in water. The products constitute a complex mixture of mono- and polynuclear hydroxo species. 

Numerous d cobalt(III) complexes are known and have been studied extensively. Most of these complexes are octahedral in shape. Tetrahedral, planar and square antiprismatic complexes of cobalt(lII) are also known, but there are very few. The most common ligands are ammonia, ethylenediamine and water. Halide ions, nitro (NO2) groups, hydroxide (OH ), cyanide (CN ), and isothiocyanate (NCS ) ions also form Co(lII) complexes readily. Numerous complexes have been synthesized with several other ions and neutral molecular hgands, including carbonate, oxalate, trifluoroacetate and neutral ligands, such as pyridine, acetylacetone, ethylenediaminetetraacetic acid (EDTA), dimethylformamide, tetrahydrofuran, and trialkyl or arylphosphines. Also, several polynuclear bridging complexes of amido (NHO, imido (NH ), hydroxo (OH ), and peroxo (02 ) functional groups are known. Some typical Co(lll) complexes are tabulated below ... 

At present, there are not sufficient data available for the change of enthalpy by formation of hydroxo complexes which are further complicated by precipitation and polynuclear behaviour. However, some of Julius Thomsen s calorimetric measmrements seem to indicate that heat frequently is evolved with hard central atoms in contrast to formation of the corresponding fluoro complexes. On the other hand, sulphate and carboxylates behave like F according to Ahrland. 

Studies on bidentate and higher dentate amine ligands have indicated that complex formation with silver(I) was rather more complicated than for monodentate amines. Polynuclear, protonated and even hydroxo complexes have been postulated to occur in solution. 

Recently the stability of silver(I) complexes of iV-methyl-substituted 4-methyldiethylenetriamines has been investigated by potentiometric pH and pAg measurements. Besides mononuclear complexes, polynuclear and protonated complexes were formed. Evidence of hydroxo complexes was also presented.48... 

Hydrolysis of polynuclear hydroxo-bridged chromium (III) complexes in concentrated solutions of strong acid yields the corresponding mononuclear species. Such cleavage reactions are fast in comparison with the hydrolysis in dilute acid and proceed with retention of configuration of the mononuclear entities. A few representative examples are shown in Eqs. (46)-(49) (40, 42,161, 252). 

The cleavage of polynuclear hydroxo-bridged rhodium(III) and iridium(III) complexes into the corresponding mononuclear fragments has been reported in only a few instances, but the well-established tendency of mononuclear complexes of these metal ions to undergo substitution reactions with retention of configuration indicates the possibility of analytical and synthetic applications such as described above for chromium (III). 

Simple multivalent cations such as Mg2+ and Al + are ineffective in charge reversal for coagulation (62) experiments and in activation for froth flotation (35). The related positive hydroxo complexes do function as coagulants and flotation activators and hence must be specifically adsorbed. By knowing the appropriate hydrolysis constants and the most effective species (in flotation systems apparently M+z(OH)Vi and in coagulation, polynuclear complexes), one can predict the pH range in which each cation can be expected to adsorb strongly. 

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