Lewis-type acid-base interactions

In his view, an acid is any species that, because of the presence of an incomplete electronic grouping, can accept an electron pair to give rise to a dative or coordination bond. Conversely, a base is any species that possess a nonbonding electron pair that can be donated to form a dative or coordination bond. The Lewis-type acid-base interaction can be depicted as follows B +A = +6 A. This definition is completely independent from water as the reaction medium, and is more general than the former one. 

Gritzner (1997) has recalled Ahrland s caution that a soft-soft interaction is not as simple as the hard-hard interaction as described by Lewis, that is, donation of share of an electron pair of the base to a vacant orbital on the acid to form a o bond, but it may also involve n back-donation from the acid to the base. There is a difference in kind as well as strength of interaction. Nevertheless, many authors have found it convenient to consider all Lewis-type acid-base reactions as lying on a spectrum from fully hard, exemplified by hydrogen bond formation, to very soft, typified by mercury-sulfur bonding, and have endeavored to construct scales of hardness or softness. They are considered in the next section. 

Acid-base interactions in the most general Lewis sense occur whenever an electron pair from one of the participants is shared in the formation of a complex, or an adduct . They include hydrogen bonding as one type of such a bond. The bond may vary from an ionic interaction in one extreme to a covalent bond in the other. Acid-base interactions and their importance in interfacial phenomena have been reviewed extensively elsewhere [35,78] and will be described only briefly here. 

The intramolecular Lewis acid-base interaction of type B is of course always in competition with an intermolecular interaction, as indicated by formula C. Again, a bulky group in a-position to X can favor the formation of monomer B. 

M. F. Lappert has therefore proposed a double Lewis acid-base interaction of type C 108-118). 

As we have seen, the Lewis theory of acid-base interactions based on electron pair donation and acceptance applies to many types of species. As a result, the electronic theory of acids and bases pervades the whole of chemistry. Because the formation of metal complexes represents one type of Lewis acid-base interaction, it was in that area that evidence of the principle that species of similar electronic character interact best was first noted. As early as the 1950s, Ahrland, Chatt, and Davies had classified metals as belonging to class A if they formed more stable complexes with the first element in the periodic group or to class B if they formed more stable complexes with the heavier elements in that group. This means that metals are classified as A or B based on the electronic character of the donor atom they prefer to bond to. The donor strength of the ligands is determined by the stability of the complexes they form with metals. This behavior is summarized in the following table. 

This definition is completely independent from water as the reaction medium and is more general than the previous ones. In terms of Lewis acidity, the Br0nsted-type acid HA is the resnlt of the interaction of the Lewis-type acid species H+ with the base A . According to the definitions given, Lewis-type acids (typically, but not only, coordinatively unsatnrated cations) do not correspond to Br0nsted-type acids (typically species with acidic hydroxyl groups). On the contrary, Lewis basic species are also Br0nsted bases. 

In coordinatively saturated metal hydrides, such as the HM(CO)s (M => Cr, Mo, W) derivatives, formation of the four-centered transition state for C02 insertion (Scheme 1) may proceed with or without CO loss and concomitant coordination ofC02 at the metal center. That is, C02 insertion may occur by means of dissociative (D) or dissociative interchange (Id) processes, or an associative interchange (Ia) process (47, 48). In either instance an acid-base interaction between the anionic hydride ligand and the electrophilic carbon center of carbon dioxide as represented in 6 may occur prior to formation of the four-centered transition state depicted in Scheme 1. An interaction of this type has been observed for these HM(CO)j derivatives with Lewis acids such as BH3 (49). 

A quantitative determination of the strength of Lewis acids to establish similar scales (Ho) as discussed in the case of protic (Br0nsted-type) superacids would be most useful. However, to establish such a scale is extremely difficult. Whereas the Brpnsted acid-base interaction invariably involves a proton transfer reaction that allows meaningful comparison, in the Lewis acid-base interaction, involving for example Lewis acids with widely different electronic and steric donating substituents, there is no such common denominator.25,26 Hence despite various attempts, the term strength of Lewis acid has no well-defined meaning. 

The hard-soft acid-base principle is not restricted to the usual types of Lewis acid-base interactions. It is a guiding principle for all types of interactions that species of similar electronic character interact best. Accordingly, we will refer to the principle as the hard-soft interaction... 

Although the bond angles are known to vary considerably depending on the metal and halogen, the arrangement is approximately tetrahedral around each metal atom. When the trihalides are dissolved in solvents that are Lewis bases, the dimers separate and complexes containing the monomer and the solvent are formed as a result of Lewis acid-base interactions. Such behavior is similar to that of borane and diborane in that the monomer, BH3, is not stable but adducts of it are. This type of behavior is illustrated in the following equations ... 

Acidic chloroaluminates (hard anions) have been utilized in catalytic reactions which need the presence of Lewis acidity (Ziegler-Natta type catalysts) and for which there is no Lewis acid-base interaction between the reaction products and aluminum chloride. Polymerization of ethylene catalyzed by (CjHjJjTiClj/Alj. Me3Cl3 complexes has been carried out in acidic chloroaluminates [20],... 

Infrared spectroscopy is a powerful method that allows the direct determination of the Bronsted centers. When pyridine (py) is adsorbed on the catalyst simultaneous determination of both types of center is possible, since it is bound to Bronsted centers in the form of a pyridinium ion through a hydrogen bond (Eq. 5-63), whereas on Lewis acid centers, adsorption occurs by a coordinative acid-base interaction (Eq. 5-64). 

---