Thiosulfates metal complexes

Uptake of small organic metal complexes over transport systems of organic metabolites may be possible, for example, of small organic acids like citrate or amino acids. However, only few examples of such processes have been studied so far. Increased uptake of cadmium by an alga has been observed in the presence of citrate and has been attributed to accidental transport of the metal-citrate complex over a citrate transporter [212]. Transport systems of inorganic anions may also play a role in metal transport. Silver uptake by algae was enhanced in the presence of thiosulfate. In this case, the silver thiosulfate complex was transported over a sulfate uptake system [213]. It remains to be demonstrated how widespread these processes may be for metal uptake in the aquatic environment [12]. 

Then the chalcogenide ions will combine with the metal ions released from the thiosulfate/selenosulfate complexes, and upon hydrolysis precipitating the corresponding chalcogenides. Due to the differences in the stabihty of the metal-chalcogenide complexes initially formed, the optimal concentrations, pH, and temperatures may vary from one chalcogenide to another. Optimal experimental conditions must be estabhshed for each system. 

In secondary wastewater treatment plants receiving silver thiosulfate complexes, microorganisms convert this complex predominately to silver sulfide and some metallic silver (see Wastes, INDUSTRIAL). These silver species are substantially removed from the treatment plant effluent at the settling step (47,48). Any silver entering municipal secondary treatment plants tends to bind quickly to sulfide ions present in the system and precipitate into the treatment plant sludge (49). Thus, silver discharged to secondary wastewater treatment plants or into natural waters is not present as the free silver ion but rather as a complexed or insoluble species. 

Stmcture (1) explains the formation of sulfur and sulfite in the presence of acid stmcture (2) is consistent with the formation of sulfide and sulfate in the presence of heavy metals. The bonding in thiosulfate complexes and the chemistry of thiosulfates are normally explained on the basis of (2) (see also... 

Thiosulfates. The ammonium, alkaU metal, and aLkaline-earth thiosulfates are soluble in water. Neutral or slightly alkaline solutions containing excess base or the corresponding sulfite are more stable than acid solutions. Thiosulfate solutions of other metal ions can be prepared, but their stabiUty depends on the presence of excess thiosulfate, the formation of complexes, and the prevention of insoluble sulfide precipitates. 

Some metal thiosulfates are inherently unstable because of the reducing properties of the thiosulfate ion. Ions such as Fe " and Cu " tend to be reduced to lower oxidation states, whereas mercury or silver, which form sulfides of low solubiUty, tend to decompose to the sulfides. The stabiUty of other metal thiosulfates improves in the presence of excess thiosulfate by virtue of complex thiosulfate formation. 

Exciting developments have occurred in the coordination chemistry of the alkali metals during the last few years that have completely rejuvenated what appeared to be a largely predictable and worked-out area of chemistry. Conventional beliefs had reinforced the predominant impression of very weak coordinating ability, and had rationalized this in terms of the relatively large size and low charge of the cations M+. On this view, stability of coordination complexes should diminish in the sequence Li>Na>K>Rb> Cs, and this is frequently observed, though the reverse sequence is also known for the formation constants of, for example, the weak complexes with sulfate, peroxosulfate, thiosulfate and the hexacyanoferrates in aqueous solutions. 

In solution this reaction is rather rapid but in the solid state autoxidation takes place much slower. Nevertheless, commercial sulfides and polysulfides of the alkali and alkali earth metals usually contain thiosulfate (and anions of other sulfur oxoacids) as impurities [6]. For all these reasons the chemistry of polysulfides is rather complex, and some of the earlier studies on polysulfides (prior to ca. 1960) are not very rehable experimentally and/or describe erroneous interpretations of the experimental results. 

We can now make sensible guesses as to the order of rate constant for water replacement from coordination complexes of the metals tabulated. (With the formation of fused rings these relationships may no longer apply. Consider, for example, the slow reactions of metal ions with porphyrine derivatives (20) or with tetrasulfonated phthalocyanine, where the rate determining step in the incorporation of metal ion is the dissociation of the pyrrole N-H bond (164).) The reason for many earlier (mostly qualitative) observations on the behavior of complex ions can now be understood. The relative reaction rates of cations with the anion of thenoyltrifluoroacetone (113) and metal-aqua water exchange data from NMR studies (69) are much as expected. The rapid exchange of CN " with Hg(CN)4 2 or Zn(CN)4-2 or the very slow Hg(CN)+, Hg+2 isotopic exchange can be understood, when the dissociative rate constants are estimated. Reactions of the type M+a + L b = ML+(a "b) can be justifiably assumed rapid in the proposed mechanisms for the redox reactions of iron(III) with iodide (47) or thiosulfate (93) ions or when copper(II) reacts with cyanide ions (9). Finally relations between kinetic and thermodynamic parameters are shown by a variety of complex ions since the dissociation rate constant dominates the thermodynamic stability constant of the complex (127). A recently observed linear relation between the rate constant for dissociation of nickel complexes with a variety of pyridine bases and the acidity constant of the base arises from the constancy of the formation rate constant for these complexes (87). 

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