Types of Lewis acids & base
Types of Lewis acids: According to Lewis concept, the following species can act as Lewis acids.
(a) Molecules in which the central atom has incomplete octet , etc.
(b) All cations are expected to act as Lewis acids since they are deficient in electrons.
(c) Molecules in which the central atom has empty d-orbitals. e.g., etc.
(d) Molecules having a multiple bond between atoms of dissimilar electro negativity e.g., .
Types of Lewis bases: The following species can act as Lewis bases.
(a) Neutral species having at least one lone pair of electrons
(b) Negatively charged species or anions
(iii) Hard and Soft principle of acids and bases: Lewis acids and bases are classified as hard and soft acids and bases. Hardness is defined as the property of retaining valence electrons very strongly. Thus a hard acid is that in which electron-accepting atom is small, has a high positive charge and has no electron which can be easily polarised or removed e.g., etc.. On the contrary, a soft acid is that in which the acceptor atom is large, carries a low positive charge or it has electrons in orbitals which are easily polarised or distorted e.g., etc..
A Lewis base which holds its electrons strongly is called hard base, e.g., , etc. on the other hand, a Lewis base in which the position of electrons is easily polarised or removed is called a soft base e.g., , etc.
In general, hard acids prefer to bind to hard bases and soft acids prefer to bind to soft bases. The bonding between hard acids and hard bases is chiefly ionic and that between soft bases and soft acids is mainly covalent.