The Lewis Acid-Base Definition

Lewis proposed his stiU broader and more useful definition of acids and bases in the late 1920s and early 1930s. Classifying acids as electron-pair acceptors and bases as electron-pair donors, he thereby liberated acid—base theory entirely from its former dependence on the presence of hydrogen. The advantage of the Lewis definition is that a larger number of reactions can be classified as acid-base than under either the Arrhenius or Bronsted-Lowry definitions. The classic example used to demonstrate the more general nature of the Lewis definition is the gas-phase reaction between boron trifluoride and ammonia, as represented in Equation (4.1)  

Note that BF3 neither provides hydrogen ions in solution nor is a proton donor as required by the Arrhenius and Bronsted-Lowry definitions, respectively. Similarly, NH3 neither provides hydroxide ions in solution nor acts as a proton acceptor. (There are other instances in which ammonia is a proton acceptor, but it does not play that role in this reaction.) Therefore, this is not an acid-base reaction under these more restricted definitions. Boron trifluoride is, however, an electron-pair acceptor and ammonia an electron-pair donor, so the reaction can be classified as acid-base under the Lewis definitions. (In a sense, the product of a reaction between a Lewis acid and a Lewis base could be called a Lewis salt. However, the more technical term for such a product is a Lewis adduct.) 

Sidgwick applied these ideas to coordination compounds. He noted that compounds such as the cobalt ammonates, described so ably by Alfred Werner s coordination theory, could also be classified as Lewis adducts. Equation (4.2) shows the formation of the hexaamminecobalt(III) cation from the Co cation and six 

Note that the metal cation is an electron-pair acceptor (Lewis acid) and each ammonia molecule is an electron-pair donor (Lewis base). The resulting coordination compound can then be thought of as a Lewis salt or adduct. 

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Jensen, W. B. (1978). The Lewis acid-base definitions a status report. Chemical Reviews, 78, 1-22. 

The Lewis acid-base definitions include reactions having nothing to do with protons. Following are some examples of Lewis acid-base reactions. Notice that the common Br0nsted-Lowry acids and bases also fall under the Lewis definition, with a proton serving as the electrophile. Curved arrows (red) are used to show the movement of electrons, generally from the nucleophile to the electrophile. 

Metal carbonyl complexes are an interesting series of coordination compounds in which the ligands are CO molecules, and in many cases the metals are present in a zero oxidation state. In these complexes, both the metal and ligand are soft according to the Lewis acid-base definitions. Although the discussion at first will be limited to the binary compounds containing only metal and CO, many mixed complexes are known that contain both CO and other ligands. 

A Lewis acid is any species capable of accepting a pair of electrons, and a Lewis base is a species with a pair of electrons available for donation. The terms acceptor and donor are also commonly used. Lewis acids include H+ and metal cations, molecules such as BF3 with incomplete octets, and ones such as SiF4 where octet expansion is possible (see Topic Cl). Any species with nonbonding electrons is potentially a Lewis base, including molecules such as NH3 and anions such as F. The Lewis acid-base definition should not be confused with the Bronsted one (see Topic E2) Bronsted bases are also Lewis bases, and H+ is a Lewis acid, but Bronsted acids such as HC1 are not Lewis acids. 

In the Lewis acid-base definition, an acid is any species that accepts a lone pair to form a new bond in an adduct. Thus, there are many more Lewis acids than other types. Lewis adds include molecules with electron-deficient atoms, molecules with polar multiple bonds, and metal cations. 

Electron-Pair Donation and the Lewis Acid-Base Definition... 

By focusing on where the proton comes from and goes to, the Brpnsted-Lowry concept expands the definition of a base to encompass a host of species that the Arrhenius definition excludes a base is any species that accepts a proton to do so, the base must have a lone electron pair. (The lone electron pair also plays the central role in the Lewis acid-base definition, as you ll see later in this chapter.)... 

ELECTRON-PAIR DONATION AND THE LEWIS ACID-BASE DEFINITION... 

The final acid-base concept we consider was developed by Gilbert N. Lewis, whose contribution to understanding the importance of valence electron pairs in molecular bonding we discussed in Chapter 9. Whereas the Brpnsted-Lowry concept focuses on the proton in defining a species as an acid or a base, the Lewis concept highlights the role of the electron pair. The Lewis acid-base definition holds that... 

The Lewis acid-base definition focuses on the donation or acceptance of an electron pair to form a new covalent bond in an adduct, the product of an acid-base reaction. Lewis bases donate the electron pair, and Lewis acids accept it. Thus, many species that do not contain El are Lewis acids. Molecules with polar double bonds act as Lewis acids, as do those with electron-deficient atoms. Metal ions act as Lewis acids when they dissolve in water, which acts as a Lewis base, to form an adduct, a hydrated cation. Many metal ions function as Lewis acids in biomolecules. 

There is an alternative view of acid-base reactions that relies on the Lewis acid-base definition but has been used to classify a wide range of organic reaction types. Cations are classified as Lewis acids, anions are Lewis bases and salts are viewed as acid-base complexes. When one writes the normal acid-base equation, certain conventions are observed. 

According to the Lewis acid-base definition, an acid is an electron pair acceptor and a base is an electron pair donor. 

Jensen, W. B. The Lewis Acid-Base Definitions A Status Report. Chem. Rev. 78,... 

Using the Lewis acid-base definition, how would you categorize a ligand How would you categorize a transition metal ion ... 

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