Stability constants complex formation, enthalpies

In contrast to stability constants, there are very few data for enthalpies and entropies of complex formation for hydroxypyranonate and hydroxypyridinonate complexes. Early studies on zinc-maltolate (190) and first-row transition metal(II) complexes of kojate (191) gave estimates of enthalpies and entropies of formation from temperature variation of stability constants, though as accurate stability constant measurements are only possible over a rather short temperature range the Aff and AS values obtained cannot be of high precision. 

The majority of reported studies of formation of cyclodextrin inclusion complexes in solution have been mainly concerned with determination of the stability constants by using equilibrium spectroscopic techniques, and the measurement of the enthalpy and entropy changes characterizing the complexation reaction. The aim of much of this work has been to determine the driving force of complex-formation. Despite the amount of research in this area, however, no general agreement has been reached, and... 

The enthalpies and entropies of formation of mono-mandelato-complexes have been determined and, in comparison with other hydroxycarboxylic acid complexes, the enthalpy order of stabilization is lactate > a-hydroxyiso-butyrate mandelate > glycolate, whereas the entropy order of stabilization is glycolate > a-hydroxyisobutyrate > mandelate > lactate. The stability constants and enthalpy of formation of mono- and di-malonate complexes have also been measured.The mono-1,1-cyclopentanedicarboxylato-complexes are less stable than the corresponding malonate species. 

A-(dihydroxybenzoyl)serine linear trimer (257) and its dimer analogue are 10" and 10 , respectively. Enthalpies of formation for the Fe complexes of the trimer and of enterobactin are reported. Linear hexapeptides and decapeptides bearing catechol units derived from dopa (dihydrox ghenylalanine) form stable complexes with Fe +, with stability constants in the range... 

At present, the correlation contains one transition metal complex, Cu(Hfacac)2. The results on this complex are very interesting and somewhat unusual for a transition metal system in that enthalpies have been obtained in a poorly solvating solvent with nonionic donors (52), instead of the t5 ical stability constant study on a metal cation in some highly polar solvent. Data from this latter type of investigation have many practical uses, but are impossible to interpret and understand. The transition metal ion complex we have studied can be incorporated into the E and C scheme using the same base parameters that are used to correlate the enthalpies of formation of all the other Lewis acid-base adducts in the scheme. 

The enthalpy and entropy of complex formation between Zn11 and picolinate and dipicolinate anions in aqueous solution have been determined by calorimetry and from formation constant data. The greater stability of the dipicolinate complex compared to the picolinate complex reflects an entropy effect, and Ais actually less favourable. These anions are well known to have a low basicity to H+ compared to their complexing ability to metals. In the present case, this probably reflects the coplanarity of the carboxylate anions and the pyridine ring, so that the oxygen atoms are in a favourable position to coordinate.800... 

Pyridines are also well known as ligands in transition metal complexes, and if the equilibrium constants for the formation of such complexes can be related to base strength, it is expected that such constants would follow the Hammett equation. The problem has been reviewed,140 and a parameter S, formulated which is a measure of the contribution of the additional stabilization produced by bond formation to the stabilization constants of complexes expressed in terms of a.141 The Hammett equation has also been applied to pyridine 1 1 complexation with Zn(II), Cd(II), and Hg(II) a,/3,y,<5-tetraphenylporphins,142 143 the a values being taken as measures of cation polarizing ability. Variation of the enthalpy of complexation for adducts of bis(2,4-pentanediono)-Cu(II) with pyridines plotted against a, however, exhibited a curved relationship.144... 

Smith, R.M. and Martell, A.E. (1987) Critical stability constants, enthalpies and entropies for the formation of metal complexes of aminopolycarboxylic acids and carboxylic acids. Sci. Total Environ., 64, 125. 

These examples and some others that are given in Table III show that for selected ions, which form strong complexes, it is possible to make unambiguous structure determinations from solution diffraction data and to obtain direct information on coordination changes that take place during the stepwise formation of complexes. Thermodynamic data provide only indirect information on these structural changes, indicated, for example, by abnormal changes in enthalpy and entropy values or in stability constants for the formation of the complexes. 

Osada studied the enthalpy and entropy changes in the complex formation between PMAA and PEO by potentiometric titration471. The fraction of the binding groups of the complex (degree of conversion, ), the stability constant (K), and thermodynamic parameters (AF°, AH0 and AS0) are related with each other by the following equations ... 

The data for bromo complexes were obtained in aqueous methanolic solutions and outer-sphere bromo complexes with K = 1.3-1.9 were obtained for Pr, Nd, Sm, which are larger than the values of Ho (0.97) and Er (0.70). Chloro and bromo complex formation in dimethyl formamide studied by titration calorimetry [122] showed the evidence for MC12+, MCC. MCI3, and MCI4 species in solutions. In the case of bromide, monobromo and dibromo inner sphere complexes have been detected. The stepwise formation constants could not be determined for iodo complexes due to the small value of enthalpies of reaction. The stability constants data obtained in DMF are given in Table 4.8. 

Of course, it is impossible to determine a a priori. If Eq. (17) is physically reasonable, it should be possible to choose a in such a way that a plot of AG (g) — AG°(g) versus DN can be represented by a fairly smooth curve, as shown in Fig. 4 for a s 0.5. This value appears reasonable low a values are highly improbable since the free Cl ion is undoubtedly a much stronger base than the coordinated Cl ion in [CoCl ] on the other hand, solvation enthalpies of the complex anions will compensate only in part, so that a is necessarily < 1. Equation (17) can be used to estimate free energies of formation or stability constants of [CoCl4] in other solvents, provided that the donicities and the values AG(sv)(Cl ) (Table V) are known. Values A( (g) — A( °(g) required for this purpose may be interpolated or extrapolated from Fig. 4. 

The stability constants of the complexes of unsaturated alcohols are higher than those of the corresponding complexes with unsaturated ammonium ions 276). The difference is due largely to the difference in enthalpy change for complex formation in the two systems and this has been attributed to the difference in solvation energies between the free ligands and the complexes. 

Solvent extraction, potentiometry, and calorimetry have been used to determine the thermodynamic parameters of the formation of the monofluoride complex of the trivalent lanthanide ions at 25°C. and an ionic strength of IM (NaClOj ). The enthalpies were all endothermic, ranging from 4.0 to 9.5 Kcal./mole consequently, the large, positive entropies, ranging from 25 to 48 cal./°C./mole, explain the high stability constants. This large entropy results from the decrease in overall water structure when the fluoride ion is complexed. The difference in the enthalpies of formation of LnF and LnAc " can possibly be explained by a difference in covalence for Ln-F and Ln-O bonds. 

Terdentate 4-amino-2,6-bis(pyridin-2-yl)-l,3,5-triazine (ADPTZ) can coordinate to Am and Ln with the formation of the 1 1 complexes. In the work [33] the thermodynamic characteristics for the complex formation of Am with ADPTZ were calculated AG=-32.9 0.6 kJ/mole, A//=-28.9 3.0 kJ/mole, A5 =14.0 10.0 J/Kmole. The stability constant equal to logy0 =5.8 O.l for [Am(ADPTZ)] complex was defined using spectrophotometry. The thermodynamic data show that the observed selectivity of the ligand arises from a difference in the enthalpies of complexation for Am and lanthanides. The geometry and electronic structure of the [M(ADPTZ)(H20)6] complexes (for M = Am, Cm, Pu) were calculated using DFT theory with no symmetry constraint. Selected distances are presented in Table 4. 

The stability constants of silver ion complexes have been evaluated by classical partition techniques (130,131) and more recently by the measurement of dissociation pressures (132,133) and the methods of gas-solid (134) and gas-liquid chromatography (135,136). The use of supported solutions of silver nitrate as a stationary phase for the separation of olefins is now quite general. Enthalpies of formation have been recorded for olefin complexes of silver borofluoride (132), for silver nitrate complexes with cyclic olefins (131) and for silver nitrate-butadiene complexes (133) the results are summarized, together with some values for the ethylene-silver ion complex, in Table XXXVIII. 

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