Complex formation reaction

Metal ion complexation rates have been studied by the T-jump method. ° Divalent nickel and cobalt have coordination numbers of 6, so they can form complexes ML with monodentate ligands L with n = 1—6 or with bidentate ligands, n = 1-3. The ligands are Bronsted bases, and only the conjugate base form undergoes coordination with the metal ion. The complex formation reaction is then... 

A further factor which must also be taken into consideration from the point of view of the analytical applications of complexes and of complex-formation reactions is the rate of reaction to be analytically useful it is usually required that the reaction be rapid. An important classification of complexes is based upon the rate at which they undergo substitution reactions, and leads to the two groups of labile and inert complexes. The term labile complex is applied to those cases where nucleophilic substitution is complete within the time required for mixing the reagents. Thus, for example, when excess of aqueous ammonia is added to an aqueous solution of copper(II) sulphate, the change in colour from pale to deep blue is instantaneous the rapid replacement of water molecules by ammonia indicates that the Cu(II) ion forms kinetically labile complexes. The term inert is applied to those complexes which undergo slow substitution reactions, i.e. reactions with half-times of the order of hours or even days at room temperature. Thus the Cr(III) ion forms kinetically inert complexes, so that the replacement of water molecules coordinated to Cr(III) by other ligands is a very slow process at room temperature. 

Complex formation reactions. These depend upon the combination of ions, other than hydrogen or hydroxide ions, to form a soluble, slightly dissociated ion or compound, as in the titration of a solution of a cyanide with silver nitrate... 

In a complex-formation reaction the equivalent is most simply deduced by writing down the ionic equation of the reaction. For example, the equivalent of potassium cyanide in the titration with silver ions is 2 moles, since the reaction is ... 

In the case of action of specific chemical forces between sorbent and solute a chemisorption process may result. This process could be regarded as a complex formation reaction and would be specific and selective, concentration dependent, possibly slow and partly irreversible. 

In 1967, Timberlake and Bridle proposed that copigmentation complex formation reactions between cyanidin and quercetin in aqueous buffered solutions took place between the colored forms of the flavylium cation (AH+) cyanidin at pH 3.0 and the quinoidal base (A) at pH 5.0. 

K. Burger, Solvation, Ionic and Complex Formation Reactions in Non-Aqueous Solvents Experimental Methods for Their Investigation, Akademiai Kiado, Budapest, 1983. 

K. Burger, Solvation, Ionic and Complex Formation Reactions in Non-Aqueous Solvents (Experimental Methods for their Investigation), Studies in Analytical Chemistry, Vol. 6, Elsevier, Amsterdam, 1983, Ch. 2 and 3 and Ch. 9, pp. 256-257. 

Ionophores such as A-23187 and X-14885A are flexible, so despite the need for conformational change, established for A-23187 by a 1H and 13C NMR solution study (556), their complex formation reactions can take place quite quickly as they can change their conformations rapidly as required for sequential bonding to the cation, and thus proceed in a series of energetically not-too-demanding steps. Formation rate constants for the Ca2+ complexes of A-23187 and X-14885A are 6 x 105 and... 

Solvent exchange reactions on metal cations are among the most simple chemical reactions a solvent molecule situated in the first coordination shell of the ion is replaced by another one, normally entering from the second shell. They are generally considered as fundamental reactions for metal ions in solution, since they constitute an important step in complex-formation reactions on metal cations. The reaction is... 

Ru(edta)(H20)] reacts very rapidly with nitric oxide (171). Reaction is much more rapid at pH 5 than at low and high pHs. The pH/rate profile for this reaction is very similar to those established earlier for reaction of this ruthenium(III) complex with azide and with dimethylthiourea. Such behavior may be interpreted in terms of the protonation equilibria between [Ru(edtaH)(H20)], [Ru(edta)(H20)], and [Ru(edta)(OH)]2- the [Ru(edta)(H20)] species is always the most reactive. The apparent relative slowness of the reaction of [Ru(edta)(H20)] with nitric oxide in acetate buffer is attributable to rapid formation of less reactive [Ru(edta)(OAc)] [Ru(edta)(H20)] also reacts relatively slowly with nitrite. Laser flash photolysis studies of [Ru(edta)(NO)]-show a complicated kinetic pattern, from which it is possible to extract activation parameters both for dissociation of this complex and for its formation from [Ru(edta)(H20)] . Values of AS = —76 J K-1 mol-1 and A V = —12.8 cm3 mol-1 for the latter are compatible with AS values between —76 and —107 J K-1mol-1 and AV values between —7 and —12 cm3 mol-1 for other complex-formation reactions of [Ru(edta) (H20)]- (168) and with an associative mechanism. In contrast, activation parameters for dissociation of [Ru(edta)(NO)] (AS = —4JK-1mol-1 A V = +10 cm3 mol-1) suggest a dissociative interchange mechanism (172). 

Kinetic studies on complex formation reactions of the tripodal tetra-mine complex [Co(Me6tren)(H20)]2+ with pyridine, 4-methylpyridine, and imidazole yielded activation parameters AH1, and AS. Activation parameters and dependences of rate constants on incoming ligand concentration indicated that the formation mechanism ranged from dissociative for the weaker and bulkier incoming ligands (py, 4-Mepy) to associative for the more basic and less bulky imidazole 2-methylimida-zole occupies an intermediate position (280). 

The general features discussed so far can explain the complexity of these reactions alone. However, thermodynamic and kinetic couplings between the redox steps, the complex equilibria of the metal ion and/or the proton transfer reactions of the substrate(s) lead to further complications and composite concentration dependencies of the reaction rate. The speciation in these systems is determined by the absolute concentrations and the concentration ratios of the reactants as well as by the pH which is often controlled separately using appropriately selected buffers. Perhaps, the most intriguing task is to identify the active form of the catalyst which can be a minor, undetectable species. When the protolytic and complex-formation reactions are relatively fast, they can be handled as rapidly established pre-equilibria (thermodynamic coupling), but in any other case kinetic coupling between the redox reactions and other steps needs to be considered in the interpretation of the kinetics and mechanism of the autoxidation process. This may require the use of comprehensive evaluation techniques. 

The following non-radical chain mechanism was proposed for the reaction in aqueous solution (for sake of simplicity, fast protolytic and complex-formation reactions are not shown) (36) ... 

A review by Brandt and van Eldik provides insight into the basic kinetic features and mechanistic details of transition metal-catalyzed autoxidation reactions of sulfur(IV) species on the basis of literature data reported up to the early 1990s (78). Earlier results confirmed that these reactions may occur via non-radical, radical and combinations of non-radical and radical mechanisms. More recent studies have shown evidence mainly for the radical mechanisms, although a non-radical, two-electron decomposition was reported for the HgSC>3 complex recently (79). The possiblity of various redox paths combined with protolytic and complex-formation reactions are the sources of manifest complexity in the kinetic characteristics of these systems. Nevertheless, the predominant sulfur containing product is always the sulfate ion. In spite of extensive studies on this topic for well over a century, important aspects of the mechanisms remain to be clarified and the interpretation of some of the reactions is still controversial. Recent studies were... 

The most controversial issue is the number and exact stoichiometries of the iron(III)-sulfito complexes formed under different experimental conditions. Earlier, van Eldik and co-workers reported the formation of a series of [Fe(SO ) ]3-2" (n = to 3) complexes and the [Fe(S03)(0H)] complex (89,91,92). The stability constants of these species were determined by evaluating time resolved rapid-scan spectra obtained from the sub-second to several minutes time domain. The cis-trans isomerization of the complexes was also considered, under feasible circumstances. In contrast, Betterton interpreted his results assuming the formation and linkage isomerization of a single complex, [Fe(SC>3)]+ (93). In agreement with the latter results, Conklin and Hoffmann also found evidence only for the formation of a mono-complex (94). However, their results were criticized on the basis that the experiments were made in 1.0 M formic acid/formate buffer where iron(III) existed mainly as formato complex(es). Although these reactions could interfere with the formation of the sulfito complex, they were not considered in the evaluation of the results (95). Finally, van Eldik and co-workers re-examined the complex-formation reactions and presented additional data in support of... 

We can now begin to see some of the implications of the theory and in which directions its applicability can be tested. In the course of these considerations one must always keep in mind that the rate of the ionogenic reaction (iii) ( left to right rate-constant k() is very small in comparison with the rates of polymerisation and of complex formation (reaction (ii)) and that the equilibrium concentration of ions is very small (see Table 1). 

The adsorption of ligands (anions and weak acids) on metal oxide (and silicate) surfaces can also be compared with complex formation reactions in solution, e.g.,... 

Most generally for a surface complex formation reaction ... 

The enhanced selectivity of the complexed transition metal cation compared to the uncomplexed aqueous form can be expressed as a gain in the stability constant of the adsorbed complex with respect to its stability constant in the solution phase (80). The complex formation reaction and corresponding stability constants of a transition metal cation M with an uncharged ligand L in both the surface (indicated by bars) and solution phase are defined as... 

The Pitzer virial coefficient method, see section 6.2.2. Methods 1 and 2 are equivalent and differ only in the form of the denominator in the Debye-Huckel term. Method 3 requires more parameters for the description of the activity factors. These parameters are not available in many cases. This is generally the case for complex formation reactions. 

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