Base properties, Lewis, solvents

The ligand of a metal complex and the solvent molecule compete with each other, as Lewis bases, to interact with the central metal ion. At the same time, the metal ion and the solvent molecule compete with each other, as Lewis acids, to interact with the ligand. Thus, the behavior of a metal complex is easily influenced by the Lewis acid-base properties of solvents. In an aprotic solvent, which is of weak acid-... 

Useful solvents must themselves resist oxidation or reduction, should dissolve suitable ionic solutes and nonelectrolytes, and in addition should be inexpensive and obtainable in high purity. Kratochvil indicated that the most potentially useful solvents are those that have a dielectric constant greater than about 25 and have Lewis-base properties. Some solvents meeting these criteria are acetonitrile, dimethyl-sulfoxide, dimethylformamide, dimethylacetamide, propylene carbonate, ethylene carbonate, formamide, sulfolane, and y-butyrolactone. Solvents of the Lewis-base type show specific solvation effects with many metal cations (Lewis acids). Thus acetonitrile functions as a Lewis base toward the silver ion. At the same time it reacts but little with the hydrogen ion. 

BrOnsted acid—base properties, Lewis acid—base propeties (EPA—EPD) etc. Such classifications make it possible to select a type of solvent which we believe will fit the reaction. Often our choice is based on assumptions as to the reaction mechanism. 

The principal factors affecting solvent-ion interactions can be classified as ion-dipole, Lewis acid-base, hydrogen-bonding, solvent structural, and steric. The solvent obviously plays a major part in these interactions. Therefore, to interpret trends in conductance data, bulk solvent properties such as viscosity and dielectric constant should be considered. Table 1 lists selected physical properties for a number of organic solvents. 

The solvent classifications used here are (1) solvents possessing both Lewis acid and Lewis base properties and a dielectric constant (D) > 25 (2) solvents possessing both Lewis acid and Lewis base properties and D < 25 (3) solvents possessing only Lewis acid properties and D> 25 (4) same as(3) but D < 25 (5) solvents possessing only Lewis base properties and D> 25 (6) same as(5)but D < 25 (7) solvents possessing negligible Lewis acid or base properties and D > 25 and (8) same as (7) but D < 25. 

Association and mobilities are related in a complex way to the bulk properties of the solvent and solute. These properties include the charge density and distribution on the ions and the Lewis base properties, the strength and nature of the solvent molecule dipole, the hydrogen-bonding capability, and the intermolecular structure of the solvent. Some correlations can be made on the basis of mobility and association trends in series such as the halides and alkali metals within a single solvent others can be drawn between solvents for a given ion. It appears that conductance measurements provide a clear measure of the sum of ion-solvent interactions, but that other techniques must be used in conjunction with conductance if assessments of individual contributions from specific factors are to be made. 

It is hoped that the terms donor and acceptor strengths will be reserved for inferences made about Lewis acid-base properties from data in the gas phase or poorly solvating solvents. This is to be contrasted with the more complex phenomena contributing to acidity and basicity. 

Acceptor number (or acceptivity), AN — is an empirical quantity for characterizing the electrophilic properties (-> Lewis acid-base theory) of a solvent A that expresses the solvent ability to accepting an electron pair of a donor atom from a solute molecule. AN is defined as the limiting value of the NMR shift, S, of the 31P atom in triethylphosphine oxide, Et3P=0, at infinite dilution in the solvent, relative to n-hexane, corrected for the diamagnetic susceptibility of the solvent, and normalized ... 

There were some attempts to estimate acid-base properties for oxide compounds, both solids and melts. The most popular of them is Lux-Flood s acid-base theory.2,3 This concept seems to be more effective for estimating the acid-base characteristics of anhydrous borates and of some promising solvents for the flux growth of borate crystals. According to Lewis-Lux s equation,... 

A review paper examines the nucleophilic properties of solvents. It is based on accumulated data derived from calorimetric measurements, equilibrium constants, Gibbs free energy, nuclear magnetic resonance, and vibrational and electronic spectra. Parameters characterizing Lewis-donor properties are critically evaluated and tabulated for a large number of solvents. The explanation of the physical meaning of polarity and discussion of solvatochromic dyes as the empirical indicators of solvent polarity are discussed (see more on this subject in Chapter 10). ... 

The model of Krygowski and Fawcett [Kr 75], developed for the description of the solvent effect and taking into account exclusively the Lewis acid-base properties of the solvent, also appears suitable for the description of the solvent dependence of the chemical shift. For example, this model reflects well the results of the Na resonance studies by Erlich et al referred to earlier [Er 70, Er 71, Gr 73]. In addition to the interaction between the sodium ion and the solvent, it also points to the dependence of the chemical shift on the concentration as a result of ion-pair formation. However, the authors themselves [Fa 76] reported that the model was unsuitable for the description of other NMR data reflecting the solvent effect. 

The important point that Lewis revealed is that though the acid-base properties of species are obviously modified by the presence or absence of a given solvent, their ultimate cause should reside in the molecular structure of the acid or base itself, and in light of the electronic theory of matter, not in a common constituent such as or OH, but in an analogous... 

The catalytic activities were found to be well correlated with the Lewis acid-base properties. UIO-66-NH2 showed the highest catalytic activity among the investigated MOFs-based materials with a maximum of 96% Ygj, obtained at 100°C, 20 bar after 4 h in the presence of chlorobenzene as solvent. Nevertheless the contribution of homogeneous reaction cannot be excluded in this case. 

The cavity term is irrelevant to the present discussion of add/base properties of the ions, the and are the Lewis acidity and basicity of the non-electrolyte, and the A j, and are the differences between the Lewis addity and basidty of the nonaqueous target solvents and those of the water source solvent (Section 3.3.2). For cations, the analogous expression is according to Marcus et al. [36] ... 

Acidic/Basic Lewis acidity/basicity determines the solvent s ability to donate or accept a pair of electrons to form a coordinate bond with solute and/or between solvent molecules. A scale for this acid/base property was proposed by Gutman (DN and AN donor and acceptor number, respectively) based on calorimetric determination. The complete proton transfer reaction with formation of protonated ions is determined by proton affinity, gas phase acidity, acid or base dissociation constants. Both concepts differ in terms of net chemical reaction. Acidity functions are not unique properties of the solvent system alone, but depend on the solute (or family of closely related solutes) with respect to which the thermodynamic tendency is measured. ... 

We have seen that Walden s fears that Lewis would deliberately eliminate the important part played by the solvent in acid-base properties were groundless. Lewises acids and bases, dissolved in suitable amphoteric solvents, have the typicaF properties of acids and bases. These typicaF properties are the properties with which we are familiar from our study of water chemistry. Now that we are beginning to branch out into other fields, we may expect to find increasingly that the electronic theory of acids and bases is the only one so far proposed that is at all adequate. 

Thus, in the modified Lewis (or Lux-Flood) concept, pure alkali halides represent the highest degree of basicity as the solvent composition changes from alkali halide-rich to alkali halide-deficient melts, the solvent becomes acidic. Acid-base properties of molten halides may be used to explain stabilization of unusually low (or high) oxidation states, the differences in stability of the same oxidation state in related melts, and the effects on coordination observed spectrally for certain metal ions. Or, restating the idea in other terms, the redox potentials depend on melt basicity. Thus, the systematic variation of melt composition is a useful technique in the arsenal of the molten salt electrochemist who is interested in the chemistry of solute species in molten salt solvents. In this respect, it is important to note that variation of temperature may be used to serve the same purpose for example, it has been shown that in neutral chloro-aluminates C1- decreases with temperature. 

We have described complex-ion formation in terms of Lewis acids and bases. Complex ions may also exhibit acid-base properties in the Bronsted-Lowry sense that is, they may act as proton donors or acceptors. Figure 24-19 represents the ionization of [Fe(H20)6] as an acid. A proton from a ligand water molecule in hexaaquairon(lll) ion is transferred to a solvent water molecule. The H2O ligand is converted to OH . 

This chapter introduces the experimental work described in the following chapters. Some mechanistic aspects of the Diels-Alder reaction and Lewis-acid catalysis thereof are discussed. This chapter presents a critical survey of the literature on solvent ejfects on Diels-Alder reactions, with particular emphasis on the intriguing properties of water in connection with their effect on rate and selectivity. Similarly, the ejfects of water on Lewis acid - Lewis base interactions are discussed. Finally the aims of this thesis are outlined. 

Finally, the solvent also interacts with sites of the Lewis acid and the Lewis base that are not directly involved in mutual coordination, thereby altering the electronic properties of the complex. For example, delocalisation of charges into the surrounding solvent molecules causes ions in solution to be softer than in the gas phase . Again, water is particularly effective since it can act as an efficient electron pair acceptor as well as a donor. 

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