Ligand-metal complexation equilibria

Despite the obvious beauty of this approach and the enormous research work dedicated to its technical realization there is no example of a covalently surface bonded hydroformylation catalyst in industry so far. The main difficulty that has been encountered is unacceptably high metal leaching from the support, mainly due to unfavorable ligand-metal complexation equilibria. Therefore, the best results have been reported for bidentate, covalently anchored ligands, with Rh leaching in the range 100-1000 ppb. Other drawbacks of the approach in... 

To utilize poly(acrylic acid) (represented as HL) as a chelating polymer in polymer-assisted ultrafiltration based removal of a divalent heavy metal (M", n = 2) present in wastewater, it is useful to consider the following polymer ligand-metal complexation equilibria ... 

This compilation of metal complex equilibrium (formation) constants and the corresponding enthalpy and entropy changes represent the authors selection of the most reliable values among those available In the literature. In many cases wide variations in published constants for the same metal complex equilibrium indicate the presence of one or more errors in ligand purity, in the experimental measurements, or in calculations. Usually, the nature of these errors is not readily apparent in the publication, and the reader is frequently faced with uncertainties concerning the correct values. In the course of developing noncritical compilations of stability constants, the authors have long felt that these wide variations in published work constitute a serious Impediment to the use of equilibrium data. Thus these critical tables were developed in order to satisfy what is believed to be an important need in the field of coordination chemistry. 

Table 3.1 summarises the influence of the diamine ligands on the equilibrium constant for binding of 3.8c to the ligand-metal ion complex (K ) and the second-order rate constant for reaction of the ternary complex (ICjat) (Scheme 3.5) with diene 3.9. 

Pyridine bases are well known as ligands in complexes of transition metals, and it might well be anticipated that the equilibrium constants for the formation of such complexes, which are likely to be closely related to the base strength, would follow the Hammett equation. Surprisingly, only very few quantitative studies of such equilibria seem to have been reported, and these only for very short series of compounds. Thus, Murmann and Basolo have reported the formation constants, in aqueous solution at 25°, of the silver(I) complexes... 

Several papers have appeared recently comparing various properties of carbonyl metal complexes substituted by various phosphines or phosphite ligands or isocyanides. Angelici and Ingemanson (4) studied the equilibrium... 

Recent reports on transition metal complexes of 2-heterocyclic thiosemicar-bazones suggest that stereochemistries adopted by these complexes often depend upon the anion of the metal salt used and the nature of the N-substituents. Further, as indicated previously, the charge on the ligand is dictated by the thione-thiol equilibrium which in turn is influenced by the solvent and pH of the preparative medium. Many of the reported complexes have been prepared in mixed aqueous solvents, often with bases added. However, there are few reports in which workers have varied the nature of their preparations to fully explore the potential diversity of these ligands. 

Dyes based on 4-phenylazo-l-naphthol (6) have been used extensively to study azo/hydrazone tautomerism since they exist as an equilibrium mixture of both the azo and hydrazone tautomers.8 However, they are of little use commercially and of no use whatsoever for metal complex azo dyes since the hydroxy group is not ortho to the azo group so these cannot act as chelating ligands. 

This procedure has been used successfully to determine the composition of many complexes in solution. It is possible to extend this method to cases where more than one complex is formed but the application is quite difficult. Like the logarithmic method, Job s method can be applied to other cases of molecular interaction and is not limited to the formation of coordination compounds. Both methods are based on the assumption that one complex is dominant in the equilibrium mixture. Numerous other methods for determining the number of metal ions and ligands in complexes have been devised, but they are beyond the introduction to the topic presented here. 

For the supported catalyst it is expected that the ligand does not leach since it is chemically bonded to the carrier. In contrast, the rhodium metal bound to the ligand is subject to leaching due to the reversible nature of the complex formation. The amount will depend on the equilibrium between rhodium dissolved in the organic phase and that bound to the ligand. When an equilibrium concentration of 10 ppb Rh is attained, the yearly loss of Rh for a 100 kton production plant will be about 1 kg Rh per year. Compared to the reactor contents of rhodium (see Table 3.9, 70 kg Rh) this would result in a loss of 1.5% of the inventory per year, which would be acceptable. 

The formation of any metal complex is a reversible reaction and at equilibrium the complex is always partially dissociated into its ligand (L) and metal ion (M) components (Scheme 5.15). The thermodynamic stability constant (K) is a measure of the extent of this... 

Exchange of complex cations. Complexation of transition metal cations with uncharged ligands such as with amines and with amino acids results in a selectivity enhancement compared to the selectivity of the aqueous metal cation (27, 65-72). Fig. 3 shows an example for the Cu(ethylenediamine) adsorption in montmorillonites of different charge density. Standard thermodynamic data for other cases are given in table IV. In all cases the free ligand concentration in equilibrium solution was... 

Speciation is a dynamic process that depends not only on the ligand-metal concentration but on the properties of the aqueous solution in chemical equilibrium with the surrounding solid phase. As a consequence, the estimation of aqueous speciation of contaminant metals should take into account the ion association, pH, redox status, formation-dissolution of the solid phase, adsorption, and ion-exchange reactions. From the environmental point of view, a complexed metal in the subsurface behaves differently than the original compound, in terms of its solubility, retention, persistence, and transport. In general, a complexed metal is more soluble in a water solution, less retained on the solid phase, and more easily transported through the porous medium. 

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