Lewis acid-base concept

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The Lewis acid-base concept is a definition of the terms acid and base that is independent of protons in the chemical sense . It was introduced by Gilbert Newton Lewis in 1923 .

Lewis acids

A Lewis acid is an electrophilic electron pair acceptor , so it can add electron pairs .

The Lewis acids include

  • Compounds with incomplete or unstable electron octets such as: B (CH 3 ) 3 , B (OH) 3 , BF 3 , AlCl 3
  • Generalization of the above rule: compounds in which an atom has no noble gas configuration . This includes metal ions such as B. Co 3+ , which in complexes such as [Co (NH 3 ) 6 ] 3+ reaches the electron configuration of krypton .
  • All metal cations that can occur as central atoms in chemical complexes . When ligands are attached , the metal cation reacts with them to form a complex. This also includes metal ions that do not have a noble gas configuration in their complexes, e.g. B. Cr 3+ .
  • Molecules with polarized double bonds, e.g. B. CO 2 , SO 3
  • Halides with "unsaturated coordination", e.g. B. SiCl 4 or PF 5

Lewis bases

A Lewis base is accordingly an electron pair donor that can provide electron pairs.

The Lewis bases therefore include compounds with atoms with at least one lone pair of electrons that can form a single bond, such as. B. ammonia , cyanide , fluoride and any other anions , water or carbon monoxide . Alkenes are also included, as their double bonds (π bonds) are quite reactive and can act in a similar way to lone pairs of electrons.

Lewis bases occur as typical ligands in metal complexes (see also complex chemistry ).

Applications of the concept

According to this concept, covalent bonds are always formed in an acid-base reaction . The product of the reaction of a Lewis acid with a Lewis base can be referred to as a Lewis acid-base adduct , coordination compound or also as an acceptor-donor complex .

In contrast to the Brønsted definition , which uses its acid constant or base constant as a measure of the strength of an acid or base , for Lewis acids and Lewis bases, in addition to a quantitative classification as strong and weak , a qualitative classification as hard and hit softly ( HSAB concept , also known as the Pearson concept). The pairing of two hard species leads to the formation of a bond that is more likely to be classified as ionic (strong), the pairing of two soft species to the formation of a rather covalent, weak bond. The energetic distance between the highest occupied molecular orbital ( HOMO ) and the lowest unoccupied molecular orbital ( LUMO ) can serve as a quantitative measure of hardness . The differentiation between soft and hard species facilitates, among other things, predictions about the position of complex formation or precipitation equilibria.

In contrast to redox reactions , in which electron acceptors and donors are also involved, in Lewis acid-base reactions there is often only a partial transfer of an electron pair with the formation of a covalent bond. H. the oxidation numbers of the reactants involved usually do not change during the reaction. However, a closer look shows that the oxidation numbers only remain unchanged if the Lewis acid has a lower electronegativity than the Lewis base.

See also


  • WB Jensen: The Lewis acid-base concepts: an overview . Wiley, New York 1980, ISBN 0471039020 .
  • Hisashi Yamamoto: Lewis acid reagents: a practical approach . Oxford University Press, New York 1999, ISBN 0198500998 .

Individual evidence

  1. Gilbert Newton Lewis: Valence and the structure of atoms and molecules . Chemical Catalog Comp., New York 1923 ( gbv.de [accessed September 9, 2017]).
  2. ^ Section Lewis bases at chemgapedia.de