Complex equilibria in solution

Because of the ability of PdCl2 in aqueous systems to catalyze the oxidation of simple olefins to the corresponding aldehyde or ketone (268), considerable attention has been devoted to the study of the nature of the complex in solution and of the kinetics of the oxidation reaction. This subject has been thoroughly reviewed (4, 556). Moiseev and coworkers 414, 467, 468) have established that the complex equilibria in solution are as represented by Eqs. (6) and (7)... 

H and 13C NMR spectra were used in a quantitative study of Al(II)-glutamate complex equilibria in solution.1095 27A1 NMR spectroscopy was able to identify species present in an equimolar Al(III)-citrate solution.1096 Complex formation equilibria were examined by 1H, 13C and 27A1 NMR spectra for the Al(III)-l-(+)-ascorbic acid system.1097 An NMR (1H, 13C) study has been nade of the Al(III) binding abilities of D-saccharinic and mucic acids.1098 27A1 NMR data were used to characterise fulvic acid-Al3+ complexes under acidic aqueous solutions.1099... 

V. I. Belevantsev and B.I. Peshchevitskii, The Investigation of Complex Equilibria in Solution, Novosibirsk, 1978 (in Russian). 

The solution behavior of K2Pt(CN)4Xo.3(H20) allows recrystallization of the complexes. Recrystallization of the bromide complex in the presence of chloride ions (even in an excess of Br ) results in the isolation of the chloride complex, Eq. 44 (86, 3646). In contrast the cation-deficient Ki.74Pt(CN)4-(H20)i.8 has not been recrystallized from solution (247). The solution behavior of Ki.74Pt(CN)4 has not been fully clarified, although another cation-deficient complex, Pt(ox)2 - , has been shown to exhibit complex equilibria in solution (244, 245), vide infra. 

The model calculations by Nagyp and Beck [Na 75] convincingly illustrate the effects which may be exerted on the complex equilibria in solution by a change in the ratio of the components of the solvent mixture. Unfortunately, the exact quantitative description even of such a comparatively simple system encounters almost insuperable difficulties. In the above case, e.g., the concentration distributions of the complexes formed in the solution have been determined by sixteen parameters, and to establish the values of these completely, or even merely to take them into consideration, is an extremely difficult task. 

Brief History of the Thermodynamics of Complex Equilibria in Solution... 

Christian, S.D. and Lane, E.H. (1975) Solvent effects on molecular complex equilibria, in Solutions and Solubilities, vol. 8 (ed. M.R.J. Dack), John Wiley Sons, Inc., New York,... 

Equilibria in Solution The stability of a protein-ligand complex in solution is measured in terms of the equilibrium constant and the standard free energy of association based on it. For association of species P and L in solution to form a complex PL, i.e., for... 

Results of relaxation measurements on spin-state equilibria in solution are available for complexes of iron(II), iron(III), and cobalt(II). The results comprise values of relaxation time r, rate constants for the forward and reverse reactions feLH activation parameters AH and AS for the two opposed... 

The Molecular Reorganization Associated with Spin-State Equilibria in Solution and the Structural Changes Accompanying Spin-State Transitions in Solid Metal Complexes... 

The deposition of a metal from a complex also involves equilibria in solution between the free metal ions, the complexed ions and the complexing agent. The formation of the complex MX2+ from the metal ion M2+ and complexing agent X is characterized by the consecutive stability constants (Section 1.4.3)... 

Nickel(II) complexes of (505) exhibit spin equilibria in solution.1355 With the bidentate analogues (506), complexes [Ni(506)2] have been isolated.1356 When Rj = Ph, the complex is tetrahedral in solution. It has a temperature independent magnetic moment of 2.75pB- When R = Me, the complex exhibits square planar-tetrahedral equilibrium in solution. Both are, however, diamagnetic in the solid state. 

Classical methods for the investigation of complex formation equilibria in solution (UV/Vis spectrometry, thermochemical and electrochemical techniques) are still in use (for an appraisal of these and other methods see, e.g., ref. 22). Examples for the determination of the ratio of metal to ligand in an Hg-protein complex by UV spectrometry are given in ref. 23, for the study of distributions of complex species of Cd in equilibria by combined UV spectrometry and potentio-metry in ref. 24 and by potentiometry alone in ref. 25, and for the combination of calorimetry and potentiometry to obtain thermodynamic data in ref. 26. 

An electrophoretic method was described by Srivastava et al. [40] to study equilibria of the cited mixed ligand complex systems in solution. Stability constants of the Zn(II) and Cd(II) complexes were 5.36 and 5.18 (log K values), respectively, at an ionic strength of 0.1 and a temperature of 35 °C. 

An ionophoretic method was described by Tewari [41] for the study of equilibria in a mixed ligand complex system in solution. This method is based on the movement of a spot of metal ion in an electric field with the complexants added in the background electrolyte at pH 8.5. The concentration of the primary ligand (nitrilo-triacetate) was kept constant, while that of the secondary ligand (penicillamine) was varied. The stability constants of the metal nitrilotriacetate-penicillamine complexes have been found to be 6.26 0.09 and 6.68 0.13 (log K values) for the Al(III) and Th(IV) complexes, respectively, at 35 °C and an ionic strength of 0.1 M. 

It is possible to resolve complex equilibria in Excel. One feasible way of setting up a spreadsheet is represented in the example of Figure 3-14. It computes the equilibrium concentrations in a titration of a solution of a metal M with a ligand L. Two complexes are formed ML and ML2. It is a 2-component 4-species system. 

The relaxation approach has played an important role in our understanding of the mechanisms of complex formation in solution (Chap. 4) 39,i4o -pjjg qj computer programs has now eased the study of multiple equilibria. For example, four separate relaxation effects with t s ranging from 100 xs to 35 ms are observed in a temperature-jump study of the reactions of Ni with flavin adenine dinucleotide (fad) (Eqn. (8.121)). The complex relaxation... 

If measurements are to be carried out at low activities (for example in studying complexation equilibria), standard solutions cannot be prepared by simple dilution to the required value because the activities would irreproducibly vary as a result of adsorption effects, hydrolysis and other side reactions. Then it is useful to use well-defined complexation reactions to maintain the required metal activity value [14, 50, 132]. EDTA and related compounds are very well suited for this purpose, because they form stable 1 1 complexes with metal ions, whose dissociation can be controlled by varying the pH of the solution. Such systems are often termed metal-ion buffers [50] (cf. also p. 77) and permit adjustment of metal ion activities down to about 10 ° m. (Strictly speaking, these systems are defined in terms of the concentration, but from the point of view of the experimental precision, the difference between the concentration and activity at this level is unimportant.)... 

Bellerby, J., Boylan, M.J., Ennis, M. and Manning, A.R. (1978) An infrared spectroscopic study of the tautomeric equilibria in solutions of tricarbonylbis (q-dienyl)isocyanidedi-iron complexes. 

Bis(aminotroponeiminato)nickel(II) complexes (124) give rise to square planar pseudotetrahedral equilibria in solution.99 1000 The amount of the paramagnetic pseudo-tetrahedral species increases as the size of the substituents R becomes greater. 

The dynamics of spin equilibria in solution are rapid. The slowest rates are those for coordination-spin equilibria, in which bonds are made and broken even these occur in a few microseconds. The fastest are the AS = 1 transitions of octahedral cobalt(II) complexes, in which the population of the e a antibonding orbital changes by only one electron these appear to occur in less than a nanosecond. For intramolecular interconversions without a coordination number change, the rates decrease as the coordination sphere reorganization increases. Thus the AS = 2 transitions of octahedral iron(II) and iron(III) are slower than the AS = 1 transitions of cobalt(II), and the planar-tetrahedral equilibria of nickel(II) are slower again, with lifetimes of about a microsecond. 

---