Hydrolysis complexes

Plutonium(III) in aqueous solution, Pu " ( 4)> is pale blue. Aqueous plutonium(IV) is tan or brown the nitrate complex is green. Pu(V) is pale red-violet or pink in aqueous solution and is beUeved to be the ion PuO Pu(VI) is tan or orange in acid solution, and exists as the ion PuO. In neutral or basic solution Pu(VI) is yellow cationic and anionic hydrolysis complexes form. Pu(VII) has been described as blue-black. Its stmcture is unknown but may be the same as the six-coordinate NpO (OH) (91). Aqueous solutions of each oxidation state can be prepared by chemical oxidants or reductants... 

Hydrolysis. Complexes formed by Pu ions with OH represent hydrolysis reactions. There is extensive interaction between Pu + and water. Pu(Ill) hydrolyzes at ca pH 7 (105) the first hydrolysis equiUbrium is as follows ... 

Preparation and chemistry of chromium compounds can be found ia several standard reference books and advanced texts (7,11,12,14). Standard reduction potentials for select chromium species are given ia Table 2 whereas Table 3 is a summary of hydrolysis, complex formation, or other equilibrium constants for oxidation states II, III, and VI. 

In order to elucidate the causes of the increased stability of the hydrolyzed cluster ions compared with the unhydrolyzed ions, further studies were made of the behaviour of [Te2X8]3 (where X = Cl,Br, or I) in solutions of hydrogen halides [43,52,80,87]. The studies were performed mainly in relation to the most stable and most readily synthesized [Tc2C18]3- ion (Fig. la) kinetic methods with optical recording were employed. The identity of the reaction products was in most cases confirmed by their isolation in the solid phase. The studies showed that the stability of the [Tc2X8]3 ions (where X = Cl, Br, or I) in aqueous solutions is determined by the sum of competing processes acid hydrolysis complex formation with subsequent disproportionation and dissociation of the M-M bonds, and oxidative addition of atmospheric oxygen to the Tc-Tc multiple bond. 

Relying on concepts such as hydrolysis, complexation, and acid-base properties, the approach embodied in this advanced mechanism is intriguing and demands further investigation in the context of electroless reactions, perhaps being co-opted into other mechanisms as the experimental conditions and results dictate. A possible drawback to the mechanism of surface incipient hydrous oxide mediators is their suggested low concentration, perhaps as low as 0.1% surface coverage [73], In practical electroless... 

Despite the extremely low concentrations of the transuranium elements in water, most of the environmental chemistry of these elements has been focused on their behavior in the aquatic environment. One notes that the neutrality of natural water (pH = 5-9) results in extensive hydrolysis of the highly charged ions except for Pu(V) and a very low solubility. In addition, natural waters contain organics as well as micro- and macroscopic concentrations of various inorganic species such as metals and anions that can compete with, complex, or react with the transuranium species. The final concentrations of the actinide elements in the environment are thus the result of a complex set of competing chemical reactions such as hydrolysis, complexation, redox reactions, and colloid formation. As a consequence, the aqueous environmental chemistry of the transuranium elements is significantly different from their ordinary solution chemistry in the laboratory. 

Formation of the N02 complex 36 (Scheme 1) upon addition of NaN02 to [(l)FeBr]Br in methanol is immediate. The product has been fully characterized by IR, NMR, UV/Vis spectroscopies and other methods. Attempts to obtain single crystals of 36, however, revealed a remarkable transformation into the nitrosyl complex (Fe(NO) 7 by slow hydrolysis. Complex 36 is diamagnetic with well-resolved NMR spectra which, similar to the spectra of the carbonyl and (Fe(NO) 6 complexes, indicate a C2V symmetrical cation in solution. The cyclic voltammogram (DMSO solution) has one quasi-reversible one-electron redox wave at —0.07 Vas the only feature, which we assign to the Fen/Fem couple of the mononuclear complex. Addition of water to methanol solutions of 36 (in sub-stoichiometric or stoichiometric amounts) under anaerobic conditions leads to the formation of the... 

When hydrolyzed in aqueous solution most metal ions form polynuclear hydrolysis complexes ... 

The analysis of solution diffraction data for these systems can be illustrated by results for indium(III) in aqueous perchlorate solutions (211). The RDFs for two concentrated solutions ( 4 M), one slightly acid containing no hydrolysis complexes and one with hydrogen ions removed, each normalized to a stoichiometric unit of volume containing one In atom, are compared in Fig. 28. The major difference between... 

Fig. 28. RDFs for an acid and a hydrolyzed indium(III) nitrate solution and the corresponding difference curve, which shows the sharp In-In interaction at 3.9 A in the polynuclear hydrolysis complexes. Theoretical peaks, calculated for the models to the right, are shown by dashed lines.

An analysis of the RDF for the acid solution shows that the In3+ ion is bonded to six water molecules at 2.17 A (Fig. 28). The same In—H20 distance is found for octahedrally coordinated In3+ in crystal structures (223, 224). According to the difference curve this coordination is not changed by the hydrolysis. A possible model for a tetranuclear complex with four octahedrally coordinated In atoms occupying the four corners of a regular tetrahedron and joined by single hydroxo bridges is shown in Fig. 28. It is consistent with the experimental data and seems to be a likely model for the hydrolysis complexes formed in solution, but has not yet been found in crystal structures. 

The stability constants indicate that solutions can be prepared in each of which one of the complexes is solely dominant (Fig. 29). Diffraction curves for two such solutions, and for an acid solution containing no hydrolysis complexes, give RDFs as shown in Fig. 30 (217). The Pb-Pb intramolecular interactions are very distinct and are clearly seen in the two hydrolyzed solutions. For the Pb4 (OH)44 + solution the single... 

Fig. 29. The percentage of lead, bound in the different hydrolysis complexes, Ph,(OH)p in a 2 M lead II) perchlorate solution, as a function of oh> which is the average number of protons removed from each hydrated lead(II) ion.

Although the metal-oxygen distances in the hydrolysis complexes are usually too weak and too irregular to give distinct features to a diffraction curve they can be observed and can sometimes be used to choose between different conceivable models for the structure. In hydrolyzed bismuth(III) solutions a dominant complex containing six Bi atoms has been shown to occur (201-204, 226,228). The octahedral arrangement of the six Bi atoms can easily be proved from the diffrac-... 

A different type of bridging occurs in hydrolysis complexes of tho-rium(IV) (219) and uranium(IV) (130). Here a distinct peak at 3.94(2) A in the hydrolyzed solutions can be ascribed to the metal-metal distances in the hydrolysis complexes. Discrete dinuclear complexes with a very similar metal-metal distance, 3.988(2) A, in which the metal atoms are joined by double hydroxo bridges have been found in crystals ofTh2(OH2)(N03)6(H20)8 (229). The same type of bridging, therefore, must occur in solution. When hydrolysis is increased, however, the number of metal-metal distances per metal atom increases beyond a value of 0.5, valid for a dinuclear complex, and larger hydrolysis complexes are obviously formed. These structures are unknown but an extensive X-ray investigation of highly hydrolyzed thorium(IV) solutions has shown that there is probably no close relation between the structures of the hydrolysis complexes in solution and the structure of thorium dioxide, which is the ultimate product of the hydrolysis process (230). 

Similar types of bridging between the metal atoms have been found for other metal ions, although complete structure determinations have not always been possible to make. Mercury(II) forms hydrolysis complexes in which three Hg2 ions are joined by an oxo group. Tin(II) forms complexes by similar bridging as found for lead(II) (Table VIII). 

Predicted trends in hydrolysis, complex formation and extraction of complexes at various experimental conditions in comparison with experimental results are summarized in Table 23. 

The effect of hydrolysis has been considered by Kolthoff and Sandell and later in more detail by Connick and McVey. The combined effects of hydrolysis, complexation with competing reagents, and formation of lower complexes with the chelating reagent have been discussed in the generalized treatment of Irving, Rossotti, and Williams. ... 

The activity coefficients in Equation (25-5), and hence the selectivity coefficient and partition ratio, depend on the concentration of electrolyte in solution and taken up by the resin. The fraction a. for metal ions depends on factors such as hydrolysis, complexation, and masking (Section 23-5). 

Since LPAS application to actinide chemistry is in its infancy, only a limited number of works are available in the published literature. Experiments hitherto performed are confined to either hydrolysis, complexation reactions with carbonate, EDTA and humate ligands and a variety of speciation works for Am(III) and to much lesser extent for U(IV), U(VI) Np(IV), Np(V), Np(VI) Pu(IV), Pu(VI). Of considerable interest is the LPAS application to the direct speciation of actinides in natural aquifer systems, where the solubility of actinides is in general very low and multi-component constituent elements as well as compounds are in much higher concentrations than actinide solubilities. The study of the chemical behaviour of actinides in such natural systems requires a selective spectroscopic method of high sensitivity. LPAS is an invaluable method for this purpose but its application to the problem is only just beginning. 

As discussed above, the presence of AI13 is usually identified via a +62.5 ppm peak in the Al-NMR spectra that arises from the central Al(0)4 site in the s-Keggin-like AI13 structure. Fu et al. (1991) and Nazar et al. (1992) noticed that additional peaks for the A1(0)4 occur in the Al-NMR spectra when AI13 solutions were reacted for extended periods of time at temperatures of 85-90°C and polymerized by titration with base. Peaks in the Al-NMR spectra appear at +64.5, +70.2, and +75.6 ppm as a peak near 0 ppm increases in intensity. The 0 ppm >eak arises from the monomeric aluminum hydrolysis complexes, mostly Al + Al(OH) at 4 < pH < 5. The peak at +64.5 ppm is weak and transient and that at 75.6 ppm only appears after extended periods of reaction. The dominant peak is at 70.2 ppm. 

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