Effect of complex formation

In a discussion of papers by Rice Harris (1954) and Harris Rice (1954), Van Wazer (1954) suggested that there could be covalent binding as well as electrostatic interaction and that cations could be held at specific sites by complex formation. 

This is a reasonable inference, because site binding is significant only with multivalent cations and strong electrostatic interactions. Under these conditions ion polarization occurs and bonds have some covalent character (Cotton Wilkinson, 1966). This is illustrated by the data of Gregor, Luttinger Loebl (1955a,b). They measured the complexation constants of poly(acrylic acid), 0 06 n in aqueous solution, with various divalent metals, which, as it so happens, are of interest to AB cements (Table 4.1). The order of stability was found to be 

Mandel Leyte (1964) found a similar order for the complexes of poly(methacrylic acid)  

Some of these divalent cations form part of the Irving-Williams series Mn, Fe, Co, Ni, Cu and Zn. Irving Williams (1953) examined the stability constants of complexes of a number of divalent ions and found that the order 

Metal ion Crystal ionic radius A Complexation constant 

Two factors are chiefly, but not exclusively, responsible for the fact that, under certain conditions, amino acids in native proteins react more rapidly than free amino acids in solution. The first and most general is the capacity of proteins to bind modification reagents at or near the functional groups of amino acid residues in orientations favorable to reaction. The reversible binary complexes formed between proteins and modification reagents prior to reaction are analogous to enzyme-substrate complexes. As a result, most site-specific modifications of native proteins probably proceed by the scheme summarized in eq. 

These two mechanistic alternatives can often be distinguished kinetic-ally. The pseudo first-order rate constant for the modification of the protein (k bs) for the schemes summarized in eqs. (4.1), (4.2) are given in eqs. (4.3), (4.4), respectively, where the 

Failure to detect a complex by this kinetic procedure is not proof that it does not form. If the concentration of the modification reagent is substantially less than K, then eq. (4.3) reduces to eq. (4.7) - an expression which is indistinguishable from a strictly bimolecular mechanism. 

One alternative approach for demonstrating the existence of a complex during the course of a modification reaction is stereochemical in nature. For example, if enantiomers of a modification reagent give either different rates of modification or different products, the importance of multiple sites of interaction between the reagent and the protein, and hence intermediate complex formation is indicated. Examples of studies of this type include the alkylation of hovine pancreatic ribonuclease and papain by a variety of haloacids (Heinrikson et al. 1965 Eisele and Wallenfels 1968). 

The simple numerical analysis below indicates the significant rate acceleration which can be achieved if a modification proceeds through the formation of a reversible complex (eq. 4.1) rather than through a 

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Table 10-10 Equilibrium model effect of complex formation on distribution of metals (all concentrations are given as — log(M)). pH = 8.0, T = 25°C. Ligands pS04 1.95 pHCOa 2.76 pCOs 4.86 pCl 0.25.

The effect of complex formation on the solubility of a solid can be observed in the home. Silver dinnerware eventually becomes discolored by an unsightly black tarnish of Ag2 S, formed from the reaction of the silver surface with small amounts of H2 S present in the atmosphere. Silver sulfide is highly insoluble in water. Commercial silver polishes contain ligands that form strong soluble complexes with Ag ions. If a tarnished serving pan is rubbed with a polish, the black tarnish dissolves, returning the silver to its brilliant shine. 

Coordinate bonds between metals and ligands result in the formation of complexes under many different types of conditions. In some cases, complexes form in the gas phase, and the number of known solid complexes is enormous. However, it is in solutions that many of the effects of complex formation are so important. For example, in qualitative analysis, AgCl precipitates when a solution of HC1 is added to one containing Ag+. When aqueous ammonia is added, the precipitate dissolves as a result of the formation of a complex,... 

The emission from [Ru(bpz)3] is quenched by carboxylic acids the observed rate constants for the process can be rationalized in terms of the protonation of the non-coordinated N atoms on the bpz ligands. The effects of concentration of carboxylate ion on the absorption and emission intensity of [Ru(bpz)3] have been examined. The absorption spectrum of [Ru(bpz)(bpy)2] " shows a strong dependence on [H+] because of protonation of the free N sites the protonated species exhibits no emission. Phosphorescence is partly quenched by HsO" " even in solutions where [H+] is so low that protonation is not evidenced from the absorption spectrum. The lifetime of the excited state of the nonemissive [Ru(Hbpz)(bpy)2] " is 1.1ns, much shorter than that of [Ru(bpz)(bpy)2] (88 nm). The effects of complex formation between [Ru(bpz)(bpy)2] and Ag on electronic spectroscopic properties have also been studied. Like bpz, coordinated 2,2 -bipyrimidine and 2-(2 -pyridyl)pyrimidine also have the... 

Figure 8.12 illustrates the effect of complex formation between protein and polysaccharide on the time-dependent surface shear viscosity at the oil-water interface for the system BSA + dextran sulfate (DS) at pH = 7 and ionic strength = 50 mM. The film adsorbed from the 10 wt % solution of pure protein has a surface viscosity of t]s > 200 mPa s after 24 h. As the polysaccharide is not itself surface-active, it exhibited no measurable surface viscosity (t]s < 1 niPa s). But, when 10 wt% DS was introduced into the aqueous phase below the 24-hour-old BSA film, the surface viscosity showed an increase (after a further 24 h) to a value around twice that for the original protein film. Hence, in this case, the new protein-polysaccharide interactions induced at the oil-water interface were sufficiently strong to influence considerably the viscoelastic properties of the adsorbed biopolymer layer. 

Complex formation removes some of the Ag+ ions from solution. As a result, to preserve the value of fCsp, more silver chloride dissolves. We can therefore conclude that the qualitative effect of complex formation is to increase the solubility of a sparingly soluble compound. 

B. Effect of Complex Formation between the Substrate and the Solvent... 

Acetylation of pyrrole is difficult because if forms a 2 1 complex with stannic chloride (29CB226). Hence, under the conditions used for the other five-membered rings (i.e., acetic anhydride in the presence of one hundredth molar equivalent of stannic chloride or iodine), no reaction occurs, and only 20% acetylation is obtained if the molar proportion of the catalyst is reduced 10-fold. The effect of complex formation also shows up in the inhibition of stannic chloride-catalyzed acetylation of fu-ran or thiophene, on addition of pyrrole (67MI4). Catalyzed acetylation of 2-cyano-, 2-formyl-, or 2-methoxycarbonylpyrrole gives mainly 4-substitution (67CJC897) indicating that the catalyst must also be coordinated with the substrate l-methyl-3-nitropyrrole acetylates only in the 5-posi-tion (57CJC21). 

Nicu VP, Neugebauer J, Baerends EJ (2008) Effects of complex formation on vibrational circular dichroism spectra. J Phys Chem A 112 6978-6991... 

Effect of Complex Formation in the EDTA/ Ca-Montmorillonite/Lead(II) Ion System... 

Red shifts were also observed to be induced by uncharged solute molecules, such as phenol. These were interpreted in terms of the superimposed effects of complex formation and of solute-induced changes in the polarizability of the medium (136). It was also suggested that complex formation is inhibited in nucleophilic "protecting" solvents (e.g., ethanol), which efficiently compete with the solute on the protonated nitrogen. 

The overall effect of complex formation is to remove a hydrated metal ion from the mixture of ions in solution by displacing the equilibrium in favour of the complex, cf. the similar process in the formation of water in acid-base titrations and precipitation reactions. 

The effect of complex formation is to increase the solubility in proportion to the square root of the decreasing fraction of metal ion in the uncomplexed form. Similar considerations may be applied to more involved types of precipitates. 

Derive an equation for the effect of complex formation on the potential of the half-reaction Max + ne M j for a case in which Max forms a series of complexes MX, MX2,... but M ed forms no complex with X. 

E. A. Moelwyn-Hughes and A. Sherman. J. Chem. Soc. 1936, 101-10. Reaction kinetics effect of complex formation. 

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