Lewis acids and bases

The Lewis acid - base concept is a definition of acid and base, which is independent of protons. It was introduced in the first half of the 20th century by Gilbert Newton Lewis.

Lewis acids

A Lewis acid is an electron pair electrophilic, that can attach electron pairs.

Among the Lewis acids

• Compounds with incomplete or unstable electron octet as: B ( CH3) 3, B (OH ) 3, BF3, AlCl3
• Generalization of the above rule: compounds in which one atom has no noble gas configuration. This includes metal ions such as Co3 , in the obtained complexes such as [Co ( NH3) 6] 3 , the electron configuration of krypton.
• All metal cations that may occur as central atoms in chemical complexes. In the addition of ligand, the metal cation reacts with these to the complex. This also includes metal ions having no noble gas configuration in their complexes, for example, Cr3 .
• Molecules with polarized double bonds, such as CO2, SO3
• Halides with " coordinatively unsaturated ", such as SiCl4 or PF5

Lewis bases

A Lewis base is an electron pair donor accordingly, which can provide electron pairs available.

The Lewis bases thus include compounds with atoms having at least one lone pair of electrons which can form a single bond, such as ammonia, alkenes, cyanide, fluoride, and any other anions, water, or carbon monoxide.

Lewis bases occur as typical ligands in metal complexes (see also Coordination Chemistry ).

Applications of the concept

In an acid -base reaction according to this concept always covalent bonds are formed. The reaction product of a Lewis acid with a Lewis base may be referred to as a Lewis acid - base adduct, coordination compound, or an acceptor - donor complex.

In contrast to the definition according to Brønsted, when used as a measure of the strength of an acid or base their acidity constant or its base constant, one strong and weak need for Lewis acids and Lewis bases in addition to a quantitative classification after additionally a qualitative classification by hard and soft hit ( HSAB concept, also known as the Pearson concept). The pairing of two soft or two hard species leads to the formation of an ionic rather than (strong) unclassifiable bond, the pairing of a hard and a soft species to form a more covalent, weak binding. As a quantitative measure of the hardness, the energy gap between the highest occupied molecular orbital ( HOMO) and the lowest unoccupied molecular orbital (LUMO ) are used. The differentiation in soft or hard species facilitates, among other predictions about the location of complex formation or precipitation equilibria.

In contrast to redox reactions in which electron and donors are also involved, is with the Lewis acid -base reactions are often only a partial transfer of an electron pair to form a covalent bond instead, i.e., the oxidation states of the reactants involved in change in the reaction does not usually. A closer look, however, shows that the oxidation numbers only remain unchanged when the Lewis acid a lower electronegativity than the Lewis base.

View

After the Lewis acid - base concept or the redox reaction to acids can be defined as substances that cleave cations or anions or electrons can absorb. Thus, the oxidizing agent or electron acceptor are and will thus be reduced in a chemical reaction. There are thus electrophiles, and correspond to electrochemically positive ( anode). Then the reciprocal assumptions apply to bases.