Stability of metal complexes in solution

EFFECT OF THE SOLVENT ON THE STRUCTURES AND STABILITIES OF METAL COMPLEXES IN SOLUTION... 

The stability of metal complexes in solution is one important aspect of the solution chemistry of metals. The structure of the solvent, the formulas and nature of the hydration spheres of the solute species present, and the reactions between species and their neighbors and the equilibria resulting therefrom have interested scientists for many years. A great deal of information has been gathered, and elaborate theories have been developed to explain the experimental data. In spite of all this effort, solution chemistry remains a challenging research field. Basic information, such as reliable stability constants for many species, especially the unstable and the very stable, is not yet available. 

Kiss, T. Lazar, 1. "Structure and stability of metal complexes in solution". Chapter 9 in "Aminophosphonic and Aminophosphinic Acids. Chemistry and Biological Activity", V.P. Kukhar, H.R. Hu n edits., Wiley, NY, 2000. 

Empirical approaches to predict the stability constants of metal complexes in solution (mostly in water) are discussed in the book by Kumok11 and in reviews by Hancock and Martell,9 Dimmock et al.,7 Hancock,8 and Hay.10 Generally, two types... 

Fig. 15 Position of alkali metal ions with respect to the six oxygen atoms of 18-crown-6, 8, which are linked by segments, in the corresponding complexes. Values in A give the distance between the metal and the mean least-squares plane of the six oxygen atoms of the ligand. These values are related to the stability of the complexes in solution the lower the distance, the higher the complexation constant [31, 33, 34]...

Job s method, or the method of continuous variation, was first published in 1928 as a means for determining the nature of metal complexes in solution. This spectrophotometric method is particularly useful for identifying metal complexes that, for stability reasons, may be difficult to isolate as a solid from solution. We will use this method to investigate the solution structure of the [Ni(salpd)] complex. 

BAN/KAD] Banerjea, D., Kaden, T. A., Sigel, H., Enhanced stability of ternary complexes in solution through the participation of heteroaromatic N bases. Comparison of the coordination tendency of p3n"idine, imidazole, ammonia, acetate, and hydrogen phosphate toward metal ion nitrilotriacetate complexes, Inorg. Chem., 20, (1981), 2586-2590. Cited on page 330. 

It is believed [1135,1136] that the decomposition of metal complexes of salicyaldoxime and related ligands is not initiated by scission of the coordination bond M—L, but by cleavage of another bond (L—L) in the chelate ring which has been weakened on M—L bond formation. Decomposition temperatures and values of E, measured by several non-isothermal methods were obtained for the compounds M(L—L)2 where M = Cu(II), Ni(II) or Co(II) and (L—L) = salicylaldoxime. There was parallel behaviour between the thermal stability of the solid and of the complex in solution, i.e. Co < Ni < Cu. A similar parallel did not occur when (L—L) = 2-indolecarboxylic acid, and reasons for the difference are discussed... 

The stability constants of several carbonato complexes of metal ions in solution has been compiled (3). Several recent publications have dealt with the isolation and characterization of carbonato complexes... 

These factors may now be considered the basis for the differences in solution stabilities of metal complexes and chelate compounds and account for the chemical phenomena summarized in the term chelate effect. Those factors that emerge as the most important in light of this paper are designated in Table VII as footnote b. 

The key to snccessfnl synthesis and isolation of transition metal peroxo complexes depends on the reaction conditions, such as temperatnre and the pH of the medinm. ° With a slight variation of the reaction pH, one can end up with compound of different stoichiometry. In the present case, for the dissolution of niobinm pentoxide, we fonnd a pH of 11 (vide experimental) to be conducive for precipitation and isolation of highly crystalline and stable niobinm peroxo com-ponnd from aqueous solution having a composition [Nb(02)4] T The reaction can be snmmarized as shown in Eqnation 6.1. In most of the peroxo metal complexes reported so far, it has been observed that in order to stabilize a peroxo metal complex in solution and then isolate it in solid state requires the presence of an ancillary ligand. However, in the present case we could isolate the... 

The bis-NHC-Ag(I) transmetallation agent generated in this reaction is unstable and significant decomposition occurs within hours. On the other hand, the resulting bis-NHC-Rh(I) complex is stable in air for months. The stability of metal-NHCs in general can be highly dependent on the N-substituents and on the identity of the metal. For example, a related bis-NHC-Rh(I) (where N-substituent is ra-butyl instead of r-butyl) decomposes in solution within a few minutes when exposed to air.10 Unlike the unstable bis-NHC-Ag(I) transmetallation agent presented here, many Ag(I) complexes of NHCs have been successfully characterized.11... 

Because ground waters, like other natural waters, are dilute solutions of many compounds, metal speciation measurements are difficult. Therefore, the metal-complexing properties of natural waters are operationally defined by many factors, including the analytic method used for speciation, the conceptual and mathematical models used to analyze the data, the range of titrant metal concentrations used, and conditions such as pH, ionic strength, and temperature. The analytical methods used to determine metal speciation all have inherent assumptions and limitations. Most published studies of metal complexation in natural waters have used one analytical method. However, confirmation of results (e.g. stability constants, ligand concentrations) by independent methods would add confidence to such results. In the present work, three independent methods were used. 

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