Solvent exchanges

Solvent Exchange.—References to solvent exchange studies will be found in previous sections. Data for the exchange between two non-aqueous solvents (S , S ) in reactions of the type [Cr(NCS)5(S )] - + [Cr(NCS)s(S )] - + S are to be 

For the cw-[M(NO)a(MeCN)J + ions (M = Mo or W), the two solvent molecules in a trans position to the nitrosyl groups are much more labile than the other two acetonitrile molecules. Solvent exchange is stereoretentive, and a mechanism involving a trigonal-bipyramidal intermediate is proposed. 

Binding of the [Co(NH3)5(H20)] + ion with an ion-exchange resin (Dowex 50w) has no effect upon the water exchange rate, whereas the pH-dependence for 0-exchange between Hg and [ReOJ ion is modified on binding to the resin.  

Methanol exchange with rm j -[Co(dmgH)2(RS02)(MeOH)] (R = Me or p-MeQHJ has also been reported.  

Solvent exchange at the [M(PR3)2(solv)2H2] cations, with M = Rh or Ir, R = phenyl or cyclohexyl, solv = acetone or acetonitrile, have been followed by proton nmr spectroscopy, with the establishment of rate constants and Arrhenius parameters. The most marked feature is the enormous acceleration induced by the trans effect of the hydride ligand, taking these reactions from the normal very slow rates characteristic of rhodium(III) and iridium(III) into the nmr time scale. Rates are considerably faster for 

Observed first-order rate constants for reaction of trans-[Rh(tn)2Cl2], tn = 1,3-propanediamine, with bromide are independent of bromide concentration, as indeed they are for all similar systems except one. The rate-determining step is replacement of the first chloride by water, with subsequent bromide anation and replacement of the second chloride quicker. Qualitative kinetic observations, including solvent effects, have been described for replacement of dialkyl sulfides by pyridine or bipyridyl in [Rh(SR2)3Cl3].  

Solvent Exchange.—Solvent exchange reactions of [Co(NHs)5(S)] + ions (S=DMSO or H2O) in DMSO or DMSO-H2O mixtures were considered previously. The 

Solvent Exchange.—Kinetic parameters for water exchange at some d and d complexes of formula [ML5(OH2)] + are given in Table 21. A comparison of 

The rate law for fluoride exchange at [TaFe] in hydrogen fluoride is 

This two-term form, normal for square-planar complexes, is extremely unusual for substitution at an octahedral complex. The tantalum(v) appears to be present in the reaction system solely as [TaF ], but of course [TaF ] is a stable anion so that parallel associative and dissociative paths for fluoride exchange represent a reasonable mechanism. Rate constants and activation parameters are listed in Table 9. The activation entropy for the ki term is entirely consistent with associative fluoride exchange via a 

Hydration of cement paste can also be stopped by diluting and removing the water present in the pores of the cement paste by a solvent. Solvent 

An ideal solvent should be miscible with water and have a relatively small molecular size such that it can enter small pores to replace the pore solution, but it should also not be too small to avoid replacement of water in the hydrates, i.e. ettringite and AFm phases. Commonly used solvents include methanol (CHjOH), ethanol (CH3CH2OH), acetone ((CHjljCO) 

Several examples of ligand exchange and replacement were considered in the previous sections and most of these will not be described further in this section. Solvent Exchange.—Recent results for the exchange of DMSO with complexes of cobalt(in), chromium(iii), and iron(ii) are summarized in Table 24. Different mechanisms are proposed for all three metal ions, an la mechanism being favoured for cobalt(m), an h mechanism for chromium(in), and a D mechanism for the [Fe(CN)8-(DMSO)] ion. The contrasting values of AS for iron(n) and chromium(m) 

A thesis is concerned with the kinetics of cis- and trans-water exchange in some aquachromium(m) complexes.  

The determination of partial molar volumes for [M(NH3)5(OH2)], M = Co, Rh, Ir, and Cr, permits an initial-state/transition-state split of the effects of the nature of the metal ion on activation volumes for water exchange. The initial state dominates, which is surprising for a series in which the transition state is believed to range from associative to dissociative. In other words, the partial molar volume of the transition state appears 

8 Coordination Numbers 6 and Above Other Inert Centers 

In this section the coverage follows mainly in order of the atomic number of the pertinent metal. However, in some cases when corresponding reactions of two metal centres (MCs) have been investigated the sub-section relates to both MCs. 

The second-order term in the rate laws for reactions of low-spin iron(II) diimine complexes with such nucleophiles as hydroxide and cyanide ions has been established as arising from a bimolecular reaction between complex and nucleophile.182 Activation volumes that were obtained for reactions of CN and OH with Fc(phcn)2 1 and Fe(bpy)3 + were in the range of +19.7 to +21.5cm3mol-1.183 Because these observations were not readily accounted for by an associative mechanism, a mechanism analogous to the Eigen-Wilkins mechanism of complex formation was introduced in which dissociative activation dominates in determining the observed activation volumes. However, subsequently it was shown that solvation 

A perspective report emphasised the key role of the application of pressure in kinetic studies in bringing clarity to understanding the mechanism of substitution reactions of cobalamins.193 The effect of various alkyl substituents in the trans position on the kinetic, thermodynamic and ground-state properties has been studied. Cobalamins featuring in these studies were cyanocobalamin (vitamin Bi2), aquacobalamin and the complex formed when the cyano or water ligand is replaced 

3 cm3 mol-1. The reverse reaction in which the coordinated cyanide is displaced by the dangling chelate is notably faster for CN(Im)Cbl, also consistent with an Id mechanism. Spectrophotometric titrations led to the result that the pAibase off equilibrium is about one unit higher than the value determined earlier for cyanocobalamin. This has its origin in the different basicities of the nucleotide unit to which the Im and DMBz groups are attached. The kinetics of the base-on/base-off reaction indicated an acid catalysed pathway. 

The dissociative nature of the reaction is evident from the magnitudes of the volumes of activation (+6.4cm3mol-1 for the substitution of ImH by H20). As in another report above the certainty of Id character is clouded by the fact that the values ofAV are composites of a precursor complex component and a contribution from the interchange step itself. A full D mechanism could not be categorically ruled out based on the properties of the system, and since the precursor formation step could contribute a negative volume term. Nevertheless a dissociative interchange mechanism was proposed as the more likely and a thorough discussion of mechanistic features, available at the time, of related systems was included. 

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Using inorganic salts as precursors, alcohol as the supercritical drying agent, and a batch process a solvent-exchange step was necessary to remove water from the gel. 

Cellulose activated with ethylenediarnine [107-15-3] is used to prepare high molecular-weight cellulose butyrate (23). Cellulose so activated has a larger measured surface area (120 m /g) than cellulose activated with acetic acid (4.8 m /g). The diamine is removed with water, followed by solvent exchange with acetic acid and butyric acid before esterification. 

R. van Eldik, ed.. Inorganic High Pressure Chemistry, Elsevier, Amsterdam, The Netherlands, 1986. High pressure coordination kinetics including solvent exchange, octahedral and four-coordinate substitution, electron transfer, photochemical, and bioinorganics are discussed. 

Panagiotopoulos et al. [16] studied only a few ideal LJ mixtures, since their main objective was only to demonstrate the accuracy of the method. Murad et al. [17] have recently studied a wide range of ideal and nonideal LJ mixtures, and compared results obtained for osmotic pressure with the van t Hoff [17a] and other equations. Results for a wide range of other properties such as solvent exchange, chemical potentials and activity coefficients [18] were compared with the van der Waals 1 (vdWl) fluid approximation [19]. The vdWl theory replaces the mixture by one fictitious pure liquid with judiciously chosen potential parameters. It is defined for potentials with only two parameters, see Ref. 19. A summary of their most important conclusions include ... 

Column performance is maintained during solvent exchange. Hhr and SuperH columns are compatible with the following solvents acetone, benzene, carbon tetrachloride, chloroform, 1-chloronaphthalene, o-chlorophenol. 

The influence of substituents on the solvolysis of benzenediazonium ions in water and in TFE is very similar. This fact suggested that the mechanism is the same in both solvents. Exchange of 3-15N-labeled 4-methoxy- and 2,4,6-trimethyl-benzene-... 

Extraction of water with methylene chloride, removal of water from extract, volume reduction to 5 mL after solvent exchange to methyl-f-butyl ether... 

Sample preparation techniques vary depending on the analyte and the matrix. An advantage of immunoassays is that less sample preparation is often needed prior to analysis. Because the ELISA is conducted in an aqueous system, aqueous samples such as groundwater may be analyzed directly in the immunoassay or following dilution in a buffer solution. For soil, plant material or complex water samples (e.g., sewage effluent), the analyte must be extracted from the matrix. The extraction method must meet performance criteria such as recovery, reproducibility and ruggedness, and ultimately the analyte must be in a solution that is aqueous or in a water-miscible solvent. For chemical analytes such as pesticides, a simple extraction with methanol may be suitable. At the other extreme, multiple extractions, column cleanup and finally solvent exchange may be necessary to extract the analyte into a solution that is free of matrix interference. 

For pesticide residue immunoassays, matrices may include surface or groundwater, soil, sediment and plant or animal tissue or fluids. Aqueous samples may not require preparation prior to analysis, other than concentration. For other matrices, extractions or other cleanup steps are needed and these steps require the integration of the extracting solvent with the immunoassay. When solvent extraction is required, solvent effects on the assay are determined during assay optimization. Another option is to extract in the desired solvent, then conduct a solvent exchange into a more miscible solvent. Immunoassays perform best with water-miscible solvents when solvent concentrations are below 20%. Our experience has been that nearly every matrix requires a complete validation. Various soil types and even urine samples from different animals within a species may cause enough variation that validation in only a few samples is not sufficient. 

Ideally, no solvent exchanges would be needed in a method, but the final extract is usually not in the same solvent as the initial extract. 

In this chapter we describe a novel, safe and efficient large-scale synthetic approach to tricycle thienobenzazepines. The key steps in the synthesis include a chemoselective hydrogenation of an aryl-nitro functionality in the presence of a 3-bromo thiophene and a subsequent palladium-catalyzed intramolecular aminocarbonylation telescoped sequentially after simple catalyst and solvent exchange. 

Samples which are difficult to extract efficiently with a few solvent exchanges can be extracted continuously at room temperature or at the boiling point of the solvent in a Soxhlet apparatus. 

II. Solvent Exchange and Ligand Substitution on Main Group Metal Ions... 

B. Solvent Exchange on Divalent Octahedral First-Row Transition Metal Ions... 

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