Ion-solvent complex

Finally, there is another aspect of the research on silylium cations. The question whether silylium cations are free or coordinated in solution requires to determine the structure of a solvated molecule. Presently, there is no experimental method available that can fulfil this task in a detailed and satisfactory manner. Questions concerning the geometry of the solvated ion, the number of solvent molecules or counterions in contact with the ion, the type of solvent-solute interactions, etc. cannot be answered directly by experiment. The information that is available on silylium ions in solution stems almost exclusively from NMR spectroscopy in form of chemical shift measurements. It is also possible to get additional information from X-ray structural analysis of ion-solvent complexes in the crystal state, however, the assumptions made to extrapolate from the solid state to solution phase can be as large as those used to extrapolate from the gas phase to solution phase. 

That means, it is possible to calculate reliable AG values from equilibrium cop-sitants for ion-solvent complex formation. If, in case of cations, the ligand (solvent. B)... 

The most comprehensive information about ion-solvent complex formation follows from potentiometric titrations and some NMR measurements. This applies to NMR studies with solutions of ions like aluminum(III), gallium(III), beryllium(II) or magnesium(II) which interact so strongly with the molecules of several dipolar solvents that the lifetime of the molecules in the solvation shell is very long. Then the solvent exchange kinetics is slow enough to observe in the NMR spectrum of the solvent separate lines for coordinated solvent molecules and for free solvent. 

Statistical mechanical approaches apply mainly to deductions about structure and are the basis of interpretations of the entropy of ions in solution and the solution s heat capacity. The entropy of a system can be calculated if the partition functions of the ions and the water molecules surrounding them are known." The partition functions (translatory, rotational, and vibrational) can be obtained from textbook material by assuming a structure of the ion-solvent complex. By comparing calculations based on various assumptions about stmcture with the values obtained from experiments, certain stmctures can be shown to be more likely (those giving rise to the calculations that match the experiment), others less probable, and some so far from the experimental values that they may be regarded as impossible. 

Now that some methods for investigating the structure of the ion-solvent complex in solution have been described, it is time to learn systematically what is known about it. One can start by considering systems that avoid the complexity of liquid water. By varying the partial pressure of water vapor while keeping it low (0.1-lOkPa), it is possible to find the equilibrium constant between water vapor and the entities represented by a number of ion-solvent aggregates, M(H20), in the gas phase (Kebarle and GodMe, 1968). 

In their first paper [384], Covington et al. consider the case where, firstly, the solvation number is the same for both solvents and, secondly, the intrinsic shielding of an ion-solvent complex varies linearly with the composition of the first solvation sheath. 

Simplest examples are prepared by the cyclic oligomerization of ethylene oxide. They act as complexing agents which solubilize alkali metal ions in non-polar solvents, complex alkaline earth cations, transition metal cations and ammonium cations, e.g. 12—crown —4 is specific for the lithium cation. Used in phase-transfer chemistry. ... 

The concentration of anionic surfactants at the sub-ppm level in natural waters and industrial waters are determined spectrophotometrically. The anionic surfactants are extracted into a nonaqueous solvent following the formation of an ion association complex with a suitable cation. 

Actinide ions form complex ions with a large number of organic substances (12). Their extractabiUty by these substances varies from element to element and depends markedly on oxidation state. A number of important separation procedures are based on this property. Solvents that behave in this way are thbutyl phosphate, diethyl ether [60-29-7J, ketones such as diisopropyl ketone [565-80-5] or methyl isobutyl ketone [108-10-17, and several glycol ether type solvents such as diethyl CeUosolve [629-14-1] (ethylene glycol diethyl ether) or dibutyl Carbitol [112-73-2] (diethylene glycol dibutyl ether). 

The principle of solvent extraction in refining is as follows when a dilute aqueous metal solution is contacted with a suitable extractant, often an amine or oxime, dissolved in a water-immiscible organic solvent, the metal ion is complexed by the extractant and becomes preferentially soluble in the organic phase. The organic and aqueous phases are then separated. By adding another aqueous component, the metal ions can be stripped back into the aqueous phase and hence recovered. Upon the identification of suitable extractants, and using a multistage process, solvent extraction can be used to extract individual metals from a mixture. 

Crown ether (Section 16.4) A cyclic polyether that, via ion-dipole attractive forces, forms stable complexes with metal ions. Such complexes, along with their accompanying anion, are soluble in nonpolar solvents. 

In the case of inorganic solutes we are concerned largely with samples in aqueous solution so that it is necessary to produce substances, such as neutral metal chelates and ion-association complexes, which are capable of extraction into organic solvents. For organic solutes, however, the extraction system may sometimes involve two immiscible organic solvents rather than the aqueous-organic type of extraction. 

An alternative to the formation of neutral metal chelates for solvent extraction is that in which the species of analytical interest associates with oppositely charged ions to form a neutral extractable species.6 Such complexes may form clusters with increasing concentration which are larger than just simple ion pairs, particularly in organic solvents of low dielectric constant. The following types of ion association complexes may be recognised. 

Those in which solvent molecules are directly involved in formation of the ion association complex. Most of the solvents (ethers, esters, ketones and alcohols) which participate in this way contain donor oxygen atoms and the coordinating ability of the solvent is of vital significance. The coordinated solvent molecules facilitate the solvent extraction of salts such as chlorides and nitrates by contributing both to the size of the cation and the resemblance of the complex to the solvent. 

Spectrophotometric methods may often be applied directly to the solvent extract utilising the absorption of the extracted species in the ultraviolet or visible region. A typical example is the extraction and determination of nickel as dimethylglyoximate in chloroform by measuring the absorption of the complex at 366 nm. Direct measurement of absorbance may also be made with appropriate ion association complexes, e.g. the ferroin anionic detergent system, but improved results can sometimes be obtained by developing a chelate complex after extraction. An example is the extraction of uranyl nitrate from nitric acid into tributyl phosphate and the subsequent addition of dibenzoylmethane to the solvent to form a soluble coloured chelate. 

Discussion. The method is based upon the complexation of boron as the bis(salicylato)borate(III) anion (A), (borodisalicylate), and the solvent extraction into chloroform of the ion-association complex formed with the ferroin. 

In any solvent system, the essential factors required for dissolution of cellulose include adequate stabihty of the electrolyte/solvent complex cooperative action of the solvated ion-pair on hydrogen bonding of cellu-... 

There is evidence, both experimental and theoretical, that there are intermediates in at least some Sn2 reactions in the gas phase, in charge type I reactions, where a negative ion nucleophile attacks a neutral substrate. Two energy minima, one before and one after the transition state, appear in the reaction coordinate (Fig. 10.1). The energy surface for the Sn2 Menshutkin reaction (p. 499) has been examined and it was shown that charge separation was promoted by the solvent.An ab initio study of the Sn2 reaction at primary and secondary carbon centers has looked at the energy barrier (at the transition state) to the reaction. These minima correspond to unsymmetrical ion-dipole complexes. Theoretical calculations also show such minima in certain solvents, (e.g., DMF), but not in water. "... 

A growing-drop method has been reported [53] for measuring interfacial liquid-liquid reactions, in which mass transport to the growing drop was considered to be well-defined and calculable. This approach was applied to study the kinetics of the solvent extraction of cupric ions by complexing ligands. 

The situation is somewhat better for the gas-phase chemistry of isolated transition-metal ions or complexes, and this area of research has received a lot of attention in the past. On the experimental side, comprehensive mass-spectrometric techniques allow for an explicit measurement of thermochemical and kinetic parameters of reactants, intermediates, and products occurring along the reaction pathways. These data can be obtained without the influence of ligands, counter ions, solvents etc. which would be a highly complicated enter-... 

Ion under test Ion-exchanger ion or complex-former ion Solvent... 

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