Cd complexes

Commercial cadion is purified by recrystn from 95% EtOH and dried. It is stable in 0.2 N KOH (in 20% aqueous EtOH) at 25°. It is a sensitive reagent for Cd, and the Cd complex has Xmax (EtOH) 475nm. [Aust Chem Inst J Proc 4 26 1937 Anal Chim Acta 19 377 1958.]... 

On the basis of the values of AS° derived in this way it appears that the chelate effect is usually due to more favourable entropy changes associated with ring formation. However, the objection can be made that and /3l-l as just defined have different dimensions and so are not directly comparable. It has been suggested that to surmount this objection concentrations should be expressed in the dimensionless unit mole fraction instead of the more usual mol dm. Since the concentration of pure water at 25°C is approximately 55.5 moldm , the value of concentration expressed in mole fractions = cone in moldm /55.5 Thus, while is thereby increased by the factor (55.5), /3l-l is increased by the factor (55.5) so that the derived values of AG° and AS° will be quite different. The effect of this change in units is shown in Table 19.1 for the Cd complexes of L = methylamine and L-L = ethylenediamine. It appears that the entropy advantage of the chelate, and with it the chelate effect itself, virtually disappears when mole fractions replace moldm . ... 

These solutions of Cd are powerful reducing agents, and after tens of microseconds all Cd has disappeared. If Cd complexes of such ligands as glycine, en, EDTA and NT A are subjected to pulse radiolysis, the Cd produced is present as a complex ... 

Various substituted sulfonylurea derivatives, partly in use as diabetes drugs, have been coordinated to Cd and Hg. Besides the IR and [H NMR spectra of several species, the structures of one each of the Cd and Hg compounds have been solved.155 In the Cd complex three ligands act as... 

Complexation of Cd with a series of polyamine macrocycles, but also related open-chain polyamines, comprising or attached to the 2,2 -bipyridine (bipy) and 1,10-phenanthroline (phen) moieties, has been studied by combined UV/vis spectrometry and potentiometry.24 Formation constants and distribution diagrams of the species present have been evaluated. As a result the thermodynamic stabilities, i.e., the formation constants, are lower for the bipy- and phen-contain-ing ligands than those for Cd complexes with aliphatic oligoaza macrocycles containing the same number of N donors. The probable reason is loss of flexibility of the ligands caused by the size and stiffness of the inserted heteroaromatic moieties. 

In a thorough work with an extensive literature survey, crystal structures, vibrational and 31P NMR spectra of several phosphine complexes of mercury(II) halides HgX2 are presented.235 In the dibenzophosphole complex (dbp)2HgBr2, Hg adopts a distorted tetrahedral coordination (rav(Hg—P) 250.2, rav(Hg—Br) 261.1pm) (cf. the related Cd complex in the previous paragraph), with slightly shorter Hg—P and Hg—Br bonds, respectively, than in the comparable... 

Subtle changes may influence the outcome of such reactions. Thus, the formation of a binuclear, halide bridged Fe-Cd complex was observed exclusively when starting from CdBr2 whereas only trinuclear Fe-Cd-Fe chains where found in the case of CdCl2 13c, d]. 

Organic complexed Cd is not important in arid soil solution. Hirsh and Banin (1990) observed 5-10% of Cd bound to organic ligands in Israeli arid soil solution. Emmerich et al. (1982) found that organic-Cd complexes constituted 1-4% of Cd in California arid soil solution. However, Villarroel et al. (1993) reported that in a California sludge-treated soil, Cd was mainly present in both free ion and organic complex forms (each accounted for 32-40% and 30-45% of total Cd in soil solution, respectively), followed by the chloride complexes (8-20%), S04-complex (3-10%), and P04-Cd complex (1.5-7.7%). The nitrate Cd complexes were the lowest. Cadmium activities and speciation is not significantly affected by P and N treatments. 

Hirsh and Banin (1990) reported that an increase in Cl concentration decreased Cd sorption due to formation of the CdCl+ ion pair. Enhanced sorption in the presence of HC03 was observed due to the formation of the CdHC03+ ion pair. Mahler et al. (1980) found that in calcareous soils treated with sewage sludge spiked with CdS04, Cd complexes were mainly Cd sulfate and carbonate complexes, and the formation of Cd-Cl complexes increased in both soils as total Cd increased. In sludge-amended California soils with pH 7-8, both Cd and Zn organic complexes and Cd phosphate complexes increase with P levels, but free Cd2+ and Zn2+ decrease with P levels (Fig. 3.4) (Villarroel et al., 1993). 

Dissolved organic carbon and plant- or microbe- produced phytosiderophores increase the solubility of most trace elements in arid and semi-arid soils. This is especially important in arid regions. High pH increases the solubility of organic matter as dissolved organic carbon in arid soils. Copper, lead and nickel have a strong tendency to form complexes, while Cd complexes are weaker. Zinc, cobalt and manganese are... 

Figure 11 proposes a structure for the PEG-a-CD complex on the basis of the conclusions derived so far. In that, a single PEG chain penetrates through... 

The kinetics of a series of guests that form 2 1 (guest CD) complexes were studied using temperature jump methods.179 183,210 The formation of the 1 1 complex was determined to be fast and to occur within the time resolution of the equipment, while... 

Table 11 Equilibrium constants and association and dissociation rate constants for alcohol/ CD complexes determined by ultrasonic relaxation at 25°C...

NMR has been used for measurements of dynamics in few examples of CD complexes, and most applications are related to slow processes, such as those observed for rotaxanes. An example of such a slow process is the threading of a-CD onto a monomeric model of ionene, which could be followed directly by measuring the intensity changes of the signals due to complexed and uncomplexed material to give an association rate constant of 0.036 M-1 s 1.204... 

The association rate constants were the same within experimental error. The dissociation rate constant for 31 was however an order of magnitude larger than that for 32. The association rate constants determined with fluorescence correlation spectroscopy were similar to the rate constants determined using temperature jump experiments (see above). However, a significant difference was observed for the dissociation rate constants where, for the 1 1 complex, values of 2.6 x 104 and 1.5 x 104s 1 were determined in the temperature jump experiments for 31 and 32, respectively.181,182 The reasons for this difference were not discussed by the authors of the study with fluorescence correlation spectroscopy. One possibility is that the technique is not sensitive enough to detect the presence of higher-order complexes, such as the 1 2 (31 CD) complex observed in the temperature jump experiments. One other possibility is the fact that the temperature jump experiments were performed in the presence of 1.0 M NaCl. 

Table 12 Association and dissociation rate constants for pyronine/CD complex at 21 °C65...

The kinetics of reactions which are influenced in a simple way by CDs may be viewed in the following manner (Bender and Komiyama, 1978 Szejtli, 1982 Tee and Takasaki, 1985). For a substrate S that undergoes an uncatalysed reaction (2) in a given medium and a catalysed reaction through a 1 1 substrate/CD complex (3), the expected variation of the observed rate constant with [CD] is given by (4). 

As pointed out above, values of KTS are obtainable from rate data without making any assumptions about the reaction mechanism. Therefore, one may use KTs and its variation with structure as a criterion of mechanism, in the same way that physical organic chemists use variations in other kinetic parameters (Brpnsted plots, Hammett plots, etc.). For present purposes, the value of Kts can be useful for differentiating between the modes of binding in the S CD complex and the TS-CD transition state, between different modes of transition state binding, and hence between different types of catalysis (Tee, 1989). 

Contrary to the above expectations, the bromination of anisole (Tee and Bennett, 1984) and of phenols (Tee and Bennett, 1988a) in the presence of a-CD is not strongly retarded, so that some form of catalysis must occur. In some cases, actual rate increases are observed in spite of the several complexations that reduce the free reactant concentrations. Analysis of the effects of substituents on the kinetics leads to the conclusion that the catalysis by a-CD most probably results from reaction of CD-bound bromine with free substrate (12a) and that the a-CD-Br2 complex is 3-31 times more reactive than free Br2 towards phenols and phenoxide ions (cf. Tee et al., 1989). For the kinetically equivalent reaction of the substrate CD complex with free bromine (12b), the rate constants (A 2 ) for phenols do not correlate sensibly with the nature and position of the substituents, and for three of the phenoxide ions they have unrealistically high values, greater than 10u m 1 s . 

For the substrates [22] reacting with /3-CD through 1 1 complexes the features are broadly similar (Table A5.10). However, for the C6, C7, C8, 2-ethylhexanoate (2EtC6), and 4-methylpentanoate (4MeC5) esters, a cleavage process involving two molecules of CD is evident at high [/3-CD] (Tee and Du, 1988, 1992). This process, which may be ascribed to attack of a second CD molecule on the 1 1 ester-CD complex (19), can be characterized by a rate constant, /cc2. 

From the rate constant kc2 and that for reaction of S-CD complex (kc), application of the Kurz approach leads to K s = kcikc2 for dissociation of the second CD molecule from the transition state in (19). Values of K rs are remarkably similar for the C6, C7, C8 and 4MeC5 esters (45, 59, 49 and 48 mM, respectively), providing strong evidence that the second CD stabilizes the transition state by binding to the aryl group of an ester molecule already bound by its alkanoate chain to the first CD [27] (Tee and Du, 1992). 

Obviously, in such cases the CD is acting as a true catalyst in esterolysis. The basic cleavage of trifluoroethyl p-nitrobenzoate by a-CD occurs by both pathways approximately 20% by nucleophilic attack and approximately 80% by general base catalysis (GBC) (Komiyama and Inoue, 1980c). The two processes are discernible because only the former leads to the observable p-nitrobenzoyl-CD. For the ester, Ks = 3.4 mM and kjka = 4.4 for the GBC route (1.25 for the nucleophilic route), and so KTS = 0.77 mM. For reaction within the ester CD complex [28], it was estimated that the effective molarity of the CD hydroxyl anion was 21-210 m (for Br0nsted /3 = 0.4 to 0.6 for GBC). Such values are quite reasonable for intramolecular general base catalysis (Kirby, 1980). 

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