Electron-pair acceptor/donor

The chemistry of Lewis acid-base adducts (electron-pair donor-acceptor complexes) has stimulated the development of measures of the Lewis basicity of solvents. Jensen and Persson have reviewed these. Gutmann defined the donor number (DN) as the negative of the enthalpy change (in kcal moL ) for the interaction of an electron-pair donor with SbCls in a dilute solution in dichloroethane. DN has been widely used to correlate complexing data, but side reactions can lead to inaccurate DN values for some solvents. Maria and Gal measured the enthalpy change of this reaction... 

For a complete quantitative description of the solvent effects on the properties of the distinct diastereoisomers of dendrimers 5 (G = 1) and 6 (G = 1), a multiparameter treatment was used. The reason for using such a treatment is the observation that solute/solvent interactions, responsible for the solvent influence on a given process—such as equilibria, interconversion rates, spectroscopic absorptions, etc.—are caused by a multitude of nonspecific (ion/dipole, dipole/dipole, dipole/induced dipole, instantaneous dipole/induced dipole) and specific (hydrogen bonding, electron pair donor/acceptor, and chaige transfer interactions) intermolecular forces between the solute and solvent molecules. It is then possible to develop individual empirical parameters for each of these distinct and independent interaction mechanisms and combine them into a multiparameter equation such as Eq. 2, "... 

It would appear that localized molecular orbital, electron-domain models will prove useful in interpretative studies of the structural chemistry of electron-pair donor-acceptor interactions. 

Gilbert Newton Lewis described covalent bonds as sharing electrons in the 1910s and the electron pair donor/acceptor theory of acids and bases in the... 

Riddle and Fowkes [21] have shown that dispersion-only liquids, such as hexane, produce a significant P shift in (C2H,s)3PO. Hence, AN values should be corrected for this dispersion effect. In many cases, this correction is quite substantial. Thus, 13.7 of the original 14.2 AN units assigned to pyridine appear to be due to dispersion rather than to specific electron-pair donor-acceptor interactions, lowering its measure of true Lewis acidity from 14.2 to 0..5. Riddle and Fowkes have found that these dispersion corrected AN values correlate well with the enthalpies of formation of the adducts formed between (C2H5)3P0 and the examined acid. They proposed to use this enthalpy as the true measure of Lewis acidity for a species, which allows to express both the DN and AN (modified AN parameter) numbers in the same units ... 

It is weU known that many materials, whether they have originally ionic, non-ionic, or molecular lattice stmctures, are transformed into the metallic state by the application of sufficiently high pressure, and indeed this can be expected to be tme of aU materials. Even quite modest increases in pressure can affect interatomic distances, spectral transitions, formal oxidation states, and many other phenomenological parameters, e.g. can increase the coordination number. Various attempts have been made in an effort to estabhsh relationships between pressure and these phenomenological parameters but none of them accounts satisfactorily for all of the observed features. This is almost certainly because of the absence, up to now, of a model which is capable of interpreting the facts without concerning itself with too detailed an interpretation of the binding forces. However, it will be shown here, after a brief survey of the present situation, that the functional approach seems to successfully provide such a model based as it is on an electron-pair donor-acceptor model of molecular interactions. 

In spite of the difficulties mentioned, an increasing number of workers have made use of dielectrometric methods in the investigation of solution structures [Ja 77b, Wi 82a, b]. All of these methods are based on the fact that intermolecular interactions between the solvent and solute also change the dielectric properties of the system. In the studies of the solvent effect, changes in the dipole moment resulting from the formation of hydrogen-bonded associates and other electron-pair donor-acceptor complexes are of particular importance [So 76]. 

Dative bonding (electron-pair donor-acceptor bonding or Lewis acid-base interactions)... 

Transition metal supramolecular chemistry based upon dative (electron-pair donor-acceptor) bonds is so extensive that its coverage would require another volume. This topic will not, therefore, be covered in this book in any more detail. Numerous examples can be identified in the various chapters of the treatise Comprehensive Organometallic Chemistry II. 

Supramolecular Self-Assembly by Dative Bonding (Electron-Pair Donor-Acceptor or Lewis Acid-Base Interactions)... 

Supramolecular self-organization in the solid state has been observed (by X-ray diffraction) for several thallium-oxygen compounds. The secondary Tl - O bonds in these compounds are rather strong and it is debatable whether these should be regarded as electron-pair donor-acceptor bonds or as secondary bonds. 

The more advanced, dsmamic coordination or functional approach considers the solute-solvent or ion-solvent interactions as a result of electron pair donor-acceptor interactions, and inductive and mesomeric effects. According to this theory, the equihbriiun between nonionized species and solvent-separated ion pairs is a function of the dsmamic electron pair acceptor and donor properties of the solvent, not simply their static polarity (73,74). Ionization may be affected by a nucleophilic attack of a donor (D) or by an electrophilic attack of an acceptor (A) directed to the substrate R—Y. The strong donor-acceptor interactions first lead to charge redistribution between and within the molecules (pile-up and spillover effects), and ultimately to the heterolytic scission of the covalent R—Y bond (eq. 8) ... 

In this rapidly growing field of asymmetric catalysis [3,4], the use of chiral Lewis acid catalysts has been well appreciated by us during the past three decades [5-7]. Although we treat transition-metal catalysis separately from Lewis acid catalysis, it should be noted that, as long as electron pair donors/acceptors are involved, the interactions between transition metals and corresponding substrates are always Lewis acid/Lewis base interactions and thus, any electron pair acceptor catalyst initiated asymmetric reaction could be regarded as chiral Lewis acid catalyzed reaction in its broadest sense. 

---