Shifting complexation equilibria

Stratifying water systems for selective extraction of thiocyanate complexes of platinum metals have been proposed. The extraction degree of mthenium(III) by ethyl and isopropyl alcohols, acetone, polyethylene glycol in optimum conditions amounts to 95-100%. By the help of electronic methods, IR-spectroscopy, equilibrium shift the extractive mechanism has been proposed and stmctures of extractable compounds, which contain single anddouble-chai-ged acidocomplexes [Rh(SCN)J-, [Ru(SCN)J, [Ru(SCN)J -have been determined. Constants of extraction for associates investigated have been calculated. 

Scheme VIII has the form of Scheme II, so the relaxation time is given by Eq. (4-15)—appjirently. However, there is a difference between these two schemes in that L in Scheme VIII is also a participant in an acid-base equilibrium. The proton transfer is much more rapid than is the complex formation, so the acid-base system is considered to be at equilibrium throughout the complex formation. The experiment can be carried out by setting the total ligand concentration comparable to the total metal ion concentration, so that the solution is not buffered. As the base form L of the ligand undergoes coordination, the acid-base equilibrium shifts, thus changing the pH. This pH shift is detected by incorporating an acid-base indicator in the solution.

It is noteworthy that all these reactions are reversible, but under reducing conditions the equilibrium shifts to the right-hand side, and under oxidizing conditions it shifts to the left-hand side. For all resulting binuclear ions at rather low concentration in solutions, reactions take place, accompanied by the rupture of the M-M bond and the formation of unstable mononuclear complexes, which are readily oxidized even by the solvent. At high concentrations of binuclear ions cycloaddition reactions of multiple M-M bonds (these reactions are considered in section 3.5) occur to form stable, under the given conditions, polynuclear clusters. 

R = H, X = S, A = Et3N and Py). In solution the former is mainly in an ionic form the latter exists as a complex. The basicity of the amine is assumed to be important. Triethylamine is a stronger base than pyridine and the ionic form is stabilized. When the proton affinity is weak, the basicity in relation to the B(III) atom, a Lewis acid, plays an important role. This involves an equilibrium shift toward the complex. This assumption is confirmed by an easy displacement of pyridine by triethylamine. The reverse process demands more severe conditions. In the NMR spectra of the triethylamine complex the signal is shifted from 22 to 42 ppm as pyridine is added. The absence of signals of two separate forms is evidence in favor of their fast interconversion. The chemical shift of the signal in 3IP spectra is 22 ppm (EtOH), 26 ppm (Py, DMFA), and 42 ppm (EtOH, Py) for complexes with triethylamine and pyridine. 

At low temperature, this equilibrium shifts to the right, and only a small fraction of free peroxyl radicals participate in the reaction, whereas at a high temperature the equilibrium is shifted to the left, and the fraction of bound radicals is small. If we assume that at 210-260 K almost all R02 radicals are bound in complexes with amines, then it can be shown that the difference of low- and high-temperature Ee0 values is close to the enthalpy of this donor-acceptor interaction in the CTC complex AEe0 AHDA = 66.7 - 59.1 = 7.6 kJ mol-1. 

Electronic ligand effects are highly predictable in oxidative addition reactions a-donors strongly promote the formation of high-valence states and thus oxidative additions, e.g. alkylphosphines. Likewise, complexation of halides to palladium(O) increases the electron density and facilitates oxidative addition [11], Phosphites and carbon monoxide, on the other hand, reduce the electron density on the metal and thus the oxidative addition is slower or may not occur at all, because the equilibrium shifts from the high to the low oxidation state. In section 2.5 more details will be disclosed. 

The methods so far discussed involve a single discrete perturbation of the chemical system with direct observation of the attendant relaxation. An oscillating perturbation of a chemical equilibrium can also lead to a hysteresis in the equilibrium shift of the system. This effect can lead to the determination of a relaxation time. The process will obviously be more complex than with discrete perturbations, and there will be problems in the monitoring. 

The molecular composition of sulfur vapor is a complex function of temperature and pressure. Vapor pressure measurements have been interpreted in terms of an equilibrium between several molecular species (9,10). Mass spectrometric data for sulfur vapor indicate the presence of all possible Sn molecules from S2 to Sg and negligible concentrations of S9 and S10 (11). In general, octatomic sulfur is the predominant molecular constituent of sulfur vapor at low temperatures, but the equilibrium shifts toward smaller molecular species with increasing temperature and decreasing pressure. 

The Pfeiffer effect (the shift in a chiral equilibrium on the addition of an optical isomer of a different compound) of racemic [Cr(ox)3]3- has been examined using for the first time optically stable metal complexes cis-[MXY(AA)2]"+ (where M = Cr3+ or Co, AA = en or tmd and X and Y = anionic monodentate ligand). It was found that the chiral equilibrium of [Cr(ox)3]3-was always displaced in favour of its A enantiomer in the presence of A enantiomers of the cis complexes, and it is proposed that the absolute configurations of cis complexes could be inferred from the equilibrium shift induced in [Cr(ox)3], 438 Laser irradiation of an aqueous solution of racemic [Cr(ox)2(phen)] or [Cr(ox)(phen)2]+ in the presence of ( + )- or (- )-cinchonine hydrochloride rapidly shifts the chiral equilibrium in a direction opposite to that induced by the usual Pfeiffer effect in the dark.439... 

Mossbauer spectroscopy of the 57Fe nucleus has been extensively used to investigate aspects of spin equilibria in the solid state and in frozen solutions. A rigid medium is of course required in order to achieve the Mossbauer effect. The dynamics of spin equilibria can be investigated by the Mossbauer experiment because the lifetime of the excited state of the 57Fe nucleus which is involved in the emission and absorption of the y radiation is 1 x 10 7 second. This is just of the order of the lifetimes of the spin states of iron complexes involved in spin equilibria. Furthermore, the Mossbauer spectra of high-spin and low-spin complexes are characterized by different isomer shifts and quad-rupole coupling constants. Consequently, the Mossbauer spectrum can be used to classify the dynamic properties of a spin-equilibrium iron complex. 

In suitably selected cases metal complexing can change the conformational rather than the chemical equilibrium. Methyl /3-D-ribopy-ranoside in aqueous solution consists of an equilibrium between the C1(d) (ll) andlC(D) (12) forms, the former predominating (17). Only the 1C (d) form has an ax-eq-ax sequence of hydroxyl groups. On addition of calcium chloride to the solution, the equilibrium shifts in favor of the IC(d) form, as seen from the value of Jit2 which decreases from 5.4 Hz in D20 to 2.5 Hz in 2.1 M CaCl2. This corresponds to a change of the proportion of the C1(d) form from 57 to 12%. 

Ealy, Jr., "Effect of Temperature Change on Equilibrium Cobalt Complex" Chemical Demonstrations, A Sourcebook for Teachers, Vol. 1 (American Chemical Society, Washington, DC, 1988), p. 60-61. Concentrated hydrochloric acid is added to pink [Co(H20)5]2+ until blue [C0CI4]2- is formed. When heated the solution turns darker blue when cooled the solution turns pink, indicating that the reaction is endothermic. Students are asked to examine the equilibrium reaction and predict how the system will shift upon the addition of water. 

Fundamental studies on the adsorption of supercritical fluids at the gas-solid interface are rarely cited in the supercritical fluid extraction literature. This is most unfortunate since equilibrium shifts induced by gas phase non-ideality in multiphase systems can rarely be totally attributed to solute solubility in the supercritical fluid phase. The partitioning of an adsorbed specie between the interface and gaseous phase can be governed by a complex array of molecular interactions which depend on the relative intensity of the adsorbate-adsorbent interactions, adsorbate-adsorbate association, the sorption of the supercritical fluid at the solid interface, and the solubility of the sorbate in the critical fluid. As we shall demonstrate, competitive adsorption between the sorbate and the supercritical fluid at the gas-solid interface is a significant mechanism which should be considered in the proper design of adsorption/desorption methods which incorporate dense gases as one of the active phases. 

In the resting state, P-450 exists as an equilibrium mixture of a hexacoordinate low-spin Fe(Ill) complex and a high-spin pentacoordinate Fe(III) complex. On substrate binding, the equilibrium shifts to favour the high-spin pentacoordinate state as the... 

However, not only the protonating ability of IIGeCh or systems derived from it determine the addition to aromatic carbon-carbon bonds, in contrast to the behavior of other HX acids. The specific features of HGeCl3 are probably manifested at the step of the cyclohexadiene derivative formation. Energy is obviously lost during the conversion from a-complex to cyclohexadiene. The formation of the cyclohexadiene-GeCl2 molecular complex (the GeCl2 present in the reaction mixture is a result of a well-known reaction, cf. Section III) is likely to be responsible for the equilibrium shift in the direction of the cyclohexadiene. It is likely that application of some other compounds which provide such shift by complexation with cyclohexadiene will enhance the addition of other HX acids to aromatic double bonds. 

We can confirm this point of view by looking at paramagnetic shifts and rs (for water) in the complexes of supposedly different water coordination. We find that in the Ln(III) series of complexes, LnY, anomalies often occur at around Tm(III) especially. We conclude that the exact structures of Ln(III) complexes like those of Na(I), K(I) and Ca(II) can not be represented by single simple pictures and we must refer to statistical populations of complexes of different structure at equilibrium. The complexity of the structural and dynamic features of this A-subgroup chemistry has been used to functional advantage both by man in cements and plasters and by biology in shells and bones and in messengers. 

Depending on the nature of the target, either Ag or Ab is added in excess. The excess is needed to ensure that all the analyte of interest is being complexed. A large excess facilitates a shift of equilibrium toward complex formation and in some cases can compensate for reduced affinity of the system. After incubation, the mixture is analyzed by CE. A peak of an affinity complex or a peak(s) of unbound reagent(s), resolved from each other and compared with that of calibration standards, is then used for analyte quantification. 

Fig. 1.7. Correlation of 1BN chemical shift differences (Auj ) determined from the f 2 dispersion measurements with equilibrium shift differences. Chemical shift differences between free pKID and the fully bound state (A4fb) are shown as black squares, and between the encounter complex and fully bound state (AJeb) are shown as gray circles, with matching shades for the lines of best fit. Reproduced with permission from [24]...

Source of the Pfeiffer Effect. No completely satisfactory explanation has yet been set forth which accounts for all of the observations associated with the Pfeiffer effect. Dwyer and co-workers (7) have proposed a configurational activity explanation which states that the dextro and levo enantiomers of optically active, labile complexes in solution are in equilibrium (with Keq. = 1), but that in the presence of an optically active environment the equilibrium shifts in favor of one of the enantiomers, resulting in a change in optical rotation. However, this proposal does not account for the fact that the effect is observed for some labile complexes and not for others. 

The binuclear germanium-capped clathrochelate [Fe2DA03(Ge(CF3)3)2] oximehydrazonate was obtained by a template condensation of the tetradentate H2DAO ligand with IGe(CF3)3 in an aqueous solution in the presence of CaCOs (Reaction 52). The resulting intramolecular macrobicyclic compound precipitated from the reaction mixture, and the equilibrium shift due to the formation of the solid allowed one to isolate this complex in a relatively high yield [73]. 

Some believe that there are two main template effects kinetic and thermodynamic [8]. The latter is responsible for an increase in the yield of the complex with ligands formed in situ in the presence of metal ions, which bind products that result from ordinary reactions and to withdraw them from the reaction medium. These procedures are not true template reactions since they do not satisfy the above-mentioned conditions, and the metal ion causes equilibrium shift only. It is impossible to distinguish between kinetic and thermodynamic contributions to the template effect, since the coordination to the metal ion simultaneously causes both steric... 

As the equilibria are frequently complex, it is often difficult to recognize a thermodynamic template effect unequivocally. The best indication is the formation of a product in unexpectedly high yield. The principle of complementarity must be emphasized here If a ligand (host) in a product mixture is complementary to an available metal ion or neutral (guest) molecule, then this will be bound selectively, and the equilibrium shifted in its favor. 

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