Hydrolysis polynuclear

The species resulting from the hydrolysis of hydrated cations such as those mentioned here are often highly complex, containing more than one metal atom (i.e. they may be polynuclear). The description here is simplified to show the essentials of the processes. 

The enthalpies of these hydrolysis reactions have also been determined (112). Polynuclear complexes such as[(Pu02)2(OH2], [(Pu02)3 (OH) ), and [(Pu02)4(0H)y] have been inferred from potentiometric titrations (105). 

Hydroxides. Thorium (TV) is generally less resistant to hydrolysis than similarly sized lanthanides, and more resistant to hydrolysis than tetravalent ions of other early actinides, eg, U, Np, and Pu. Many of the thorium(IV) hydrolysis studies indicate stepwise hydrolysis to yield monomeric products of formula Th(OH) , where n is integral between 1 and 4, in addition to a number of polymeric species (40—43). More recent potentiometric titration studies indicate that only two of the monomeric species, Th(OH) " and thorium hydroxide [13825-36-0], Th(OH)4, are important in dilute (<10 M Th) solutions (43). However, in a Th02 [1314-20-1] solubiUty study, the best fit to the experimental data required inclusion of the species. Th(OH) 2 (44). In more concentrated (>10 Af) solutions, polynuclear species have been shown to exist. Eor example, a more recent model includes the dimers Th2(OH) " 2 the tetramers Th4(OH) " g and Th4(OH) 2 two hexamers, Th2(OH) " 4 and Th2(OH) " 2 (43). 

The pH effect in Fenton reactions is due to the Fe speciation. In highly acidic solutions containing noncoordinating species, Fe exists as Fe(H20)f. The composition as function of the pH is represented in Figure 6.3a. At increasing pH, Fe undergoes hydrolysis forming FeOH +, Fe(OH)2+, and finally FeO via binuclear, Fe2(OH)4, and polynuclear species. The aim of optimization is to avoid Fe precipitation, either bi-, poly-, or precipitated Fe oxides that are aU inactive in... 

Except at very low pH values, the hexa-aqua ferric ion, Fe(H20)63+, would then undergo a series of hydrolysis and polymerization reactions, leading progressively to more and more insoluble ferric polynuclears which would precipitate to give the geologic evidence of atmospheric oxygenation by the presence around the... 

Hydrolysis of Iron(lll) in Acid Media - Formation of Polynuclear Species... 

A review of recent advances in chromium chemistry (82) supplements earlier comprehensive reviews of kinetics and mechanisms of substitution in chromium(III) complexes (83). This recent review tabulates kinetic parameters for base hydrolysis of some Cr(III) complexes, mentions mechanisms of formation of polynuclear Cr(III) species, and discusses current views on the question of the mechanism(s) of such reactions. It seems that both CB (conjugate base) and SVj2 mechanisms operate, depending on the situation. The important role played by ionpairing in base hydrolysis of macrocyclic complexes of chromium(III) has been stressed. This is evidenced by the observed order, greater... 

Last but not least of the liquid calorimetric media are aqueous solutions used in the hydrolysis of simple and complex fluorides. Stepwise replacement of F by OH occurs, and mixed products are not unusual. Thus the BFj ion hydrolyzes to species BF (OH)l and one has to ensure that the same product composition is formed in the auxiliary heat experiments (99). The problem is accentuated when polynuclear species form, as the equilibration can be slow. The inconsistencies in the heats of alkaline hydrolysis of MoF6 and WFe found by various authors and of the enthalpy of SbF5—derived by assuming SbF5 and Sb205 dissolved in 10 M HF produced the same species in solution—illustrate the difficulties. It is as well to confirm enthalpies of higher valent fluorides obtained by hydrolysis by alternative nonaqueous methods, especially since uncertainty in the Afl (Fderived enthalpy. The advantage of hydrolysis methods, apart from the simplicity of technique, is that the heats are small and one can tolerate... 

The vanadate (1, 2), molybdate (1-5), and tungstate (1-3) systems have been described in previous reviews. Although the focus in this chapter is on more recent developments, earlier well-established knowledge is included where needed for perspective and also to present a coherent picture of the hydrolysis behavior of these oxyanions. Equilibria of mono- and polynuclear species are described and information about known structures are given. Some recent work about mixed polyoxoanions is briefly reviewed. 

At higher temperatures the monomer is the predominant species although the rate of hydrolysis to U03 is increased. U03 dissolves in uranyl solutions to give U02OH+ and polymerised hydroxo-bridged species. Polynuclear species could arise from U4+ as it hydrolyses in dilute acid solutions. Complex ions are formed with thiocyanate, phosphate, citrate and anions of other organic acids. 

As the number of reaction steps increases so, of course, does the complexity. A novel example is the six consecutive steps for hydrolysis of a cyanobridged polynuclear complex 9 ... 

Observation of the semblance of an S-shaped profile has been used to estimate the pAT n value for an acid/base pair which may be difficult to obtain directly. This may be as the result of one of the pairs being unstable (polynuclear formation or hydrolysis, for example). 

The tetraanionic [H2O Al3(/r-OH)(/r-OR)3]" [R = C(CH2C02)2C02] moiety has been achieved by the combination of (H20)9 Al(N03)3 with citric acid [101], with the synthesis establishing the existence of polynuclear complexes of a type previously postulated [102, 103], Al NMR spectroscopic studies have established that the tris(aluminium) cluster (Section 3.2.5) is not fluxional in D2O but instead retains three distinct metal environments over a wide pH range (3.0-9.0) with acid hydrolysis occurring below pH 3.0 and the resultant spectrum revealing the presence of [(H2O), (RO)Al3] and/or [(H20) (ROH) A13] " and also [(HjOlg Al] "" [101]. 

Polynuclear Complexes of Iron and their Biological Implications Table 1. Hydrolysis and Dimerization of Fe III) Chelates... 

Hydrolysis of water molecules induced by ions in aqueous solution or at the surfaces of minerals or polynuclear ions is a fundamental aspect of most dynamic processes taking place at... 

The hydrolysis of [Ni(H20)6]2+ has been studied by some authors. It has been found that polynuclear species are formed in alkaline solution, die main product being the tetranuclear cation [Ni4(OH)4]4+.1468-1471 A few solid compounds which contain nickel(II) linked only to hydroxide ions were reported, namely M2[Ni(OH)6] (M = Ba, Sr) and Na2[Ni(OH)4].l47a-1474... 

These two mechanisms, the one emphasizing the adsorption of specific often polynuclear hydrolysis products, the other emphasizing the role of polymeric species, are clearly not mutually exclusive an earlier study on thorium (IV) adsorption suggested a combined mechanism (J). 

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... 

Polynuclear complexes with hydroxide (or oxide) as bridging ligands constitute an important class of complexes. They are formed by hydrolysis of mononuclear aqua complexes of most metal ions and they therefore constitute an important aspect of the hydrolytic chemistry of metal ions. They display a chemistry which is interesting in itself, but which is also relevant in relation to applied chemistry and to biochemistry, as mentioned in Section XIII. 

Hydrolysis is strong acid in which all hydroxo bridges are cleaved, followed by identification of the various mononuclear species and a determination of their molar ratios, may provide extremely valuable information. A straightforward example is the cleavage of the tetra-nuclear species Cr4(NH3)12(OH)66+, which yields Cr(H20)63+ and cis-Cr(NH3)4(H20)23+ in a ratio of 1 3 (40). Since it could be demonstrated at the same time that the polynuclear cation does not exhibit acid base properties in the pH region for terminally coordinated water, it was concluded that the only possible structure was 6 in Fig. 1, as later confirmed by a crystal-structure analysis (41). 

Formation of hydroxo-bridged complexes by hydrolysis in aqueous solution is, not surprisingly, the most common preparative method. As a rule, such reactions give quite complex product mixtures containing species with different nuclearities, each of which may be present in many isomeric forms. The fact that most of the preparative procedures employed lead to the isolation of one single and pure isomer probably more often reflects favorable solubility properties rather than stereospecificity. In some cases ion-exchange chromatography has been used to isolate the polynuclear species, but systematic analysis of hydrolysis mixtures by this technique has been reported for only a few systems. 

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