Ligands aqueous complexes

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... 

A critical assessment of the chemical thermodynamic properties of the actinides and their compounds is presently being prepared by an international team of scientists under the auspices of the International Atomic Energy Agency (Vienna). As a result of this effort, four publications (1, 2, 3, 5) have already become available and a further ten 6-T4), including the halides (8) and aqueous complexes with Tnorganic ligands (12),... 

The ligand 6,13-dimethyl-l,4,8,ll-tetra-azacyclotetradecane-6,13-diamine coordinates as a hexadentate ligand to zinc in neutral aqueous solution. Potentiometric titrations were used to determine the stability constant for formation. The pXa values were determined for five of the six possible protonation steps of the hexamine (2.9, 5.5, 6.3, 9.9 and 11.0).697 Studies of the syn and anti isomers of 6,13-dimethyl-1,4,8, ll-tetraazacyclotetradecane-6,13-diamine reveal that they offer different shapes for metal binding, which is reflected in the stability constants for 1 1 zinc ligand ratio complexes. The selectivity of binding to the zinc ion compared to the cadmium(II) ion by both isomers is significant.698... 

Unmodified poly(ethyleneimine) and poly(vinylpyrrolidinone) have also been used as polymeric ligands for complex formation with Rh(in), Pd(II), Ni(II), Pt(II) etc. aqueous solutions of these complexes catalyzed the hydrogenation of olefins, carbonyls, nitriles, aromatics etc. [94]. The products were separated by ultrafiltration while the water-soluble macromolecular catalysts were retained in the hydrogenation reactor. However, it is very likely, that during the preactivation with H2, nanosize metal particles were formed and the polymer-stabilized metal colloids [64,96] acted as catalysts in the hydrogenation of unsaturated substrates. 

Shock E. L. and Koretsky C. M. (1995). Metal-organic complexes in geochemical processes Estimation of standard partial molal thermodynamic properties of aqueous complexes between metal cations and monovalent organic acid ligands at high pressures and temperatures. Geochim. Cosmochim. Acta, 59 1497-1532. 

Diastereomeric distributions (A A) in several tris-ligand Fe + complexes of chiral catecholamide and terephthalamide ligands have been established in solution by CD and NMR spectroscopies. The complex of (253), whose structure in the solid state was determined, exists wholly in the A conformation in aqueous solution. Weak polar interactions determine conformational preferences here. ... 

Using a nonequilibrium approach, strong binding can be studied (ligand-receptor complex) (43). However, of particular interest in ACE and MACE is the characterization of weak interactions, since the rate of complex formation and the exchange of solute between aqueous and micellar phase could be too fast to be studied with conventional structure determination methods (MS, NMR). The alternative to those methods, namely, to measure in an equilibrium state, makes MACE highly attractive. Thus, weak bond strengths (acid-base and complex/partition equilibria) are measurable. 

On the other hand, an attempt to accelerate the step of coordination of the substrate to the Cu catalyst was successful because it used the hydrophobic domain of the polymer ligand156 That was the oxidation catalyzed by polymer-Cu complexes in a dilute aqueous solution of phenol, which occurred slowly. The substrate was concentrated in the domain of the polymer catalyst and was effectively catalyzed by Cu in the domain. A relationship was found to exist between the equilibrium constant (Ka) for the adsorption of phenol on the polymer ligand and the catalytic activity (V) of the polymer-ligand-Cu complex for various polymer ligands K a = 0.21 1/mol and V = 1(T6 mol/1 min for QPVP, K a = 26 and V = 1(T4 for PVP, K a = 52 and V = 10-4 for the copolymer of styrene and 4-vinylpyridine (PSP) (styrene content 20%), and K a = 109 and V = 10-3 for PSP (styrene content 40%). The V value was proportional to the Ka value, and both Ka and V increased with the hydrophobicity of the polymer ligand. The oxidation rate catalyzed by the polymer-Cu complex in aqueous solutions depended on the adsorption capacity of the polymer domain. 

Let s derive an equation for the distribution coefficient of a metal between two phases when essentially all the metal in the aqueous phase (ag) is in the form Mn+ and all the metal n the organic phase (org) is in the form ML (Figure 23-3). We define the partition coeffi- ients for ligand and complex as follows ... 

Mixed-ligand Crm complexes have a particularly rich substitutional photochemistry in that two (or more) reaction modes are normally observed. Data for the well-studied class of acidoamine complexes are presented in Table 2. The dominant photochemical reaction for [CrX(NH3)5]2+ complexes in aqueous solution is NH3 aquation, with X- aquation occurring to a lesser extent (equation 31). In contrast, the latter pathway is the favored thermal reaction of these compounds. Such behavior again illustrates that the reactivity of ligand field excited states can differ sharply from that of the ground state. 

Many aromatic AT-oxides are commercially available. They may be prepared in good yields by addition of 1.7 mol equivalents of 35% aqueous hydrogen peroxide to a solution of the amine in glacial acetic acid at 70-80 "C.73 These ligands form complexes with nearly every element. They form 1 1 complexes with I2.73... 

At each step of the reaction the product should be isolated otherwise the final result will surely be a mixture of compounds. As an example of this type of procedure we consider an early paper by Dwyer et al.91. [Ru(bpy)Cl4] with pyridine in aqueous ethanol led to [Ru(bpy)(py)4](C104)2, which could be converted into several [Ru(bpy) (py)2(AB)]n+ compounds. Later we showed that compounds of this type, on long reflux with more bidentate ligand, yielded complexes in which the remaining pyridines were replaced93. ... 

A complex (-503 ppm) is readily formed when one acetate is replaced with the pyridyl functionality (2-Cyr). These complexes have octahedral coordination in the solid state, and unlike the complexes described above, this coordination is retained in aqueous solution [49], The ligand is complexed in a tetradentate fashion, and here, as similarly observed for the analogous amino alcohol complexes, the presence of pyridine in the coordination sphere has little influence on 51V NMR chemical shifts. Other complexes with similar tetradentate ligands have also been found to retain octahedral coordination when in aqueous medium [54],... 

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