Most polar solvents have Lewis base or donor properties resulting from lone-pair electrons. Good donor solvents inbuilt water, pyridine and ammonia, and are efficient at solvating cations and other Lewis acids.
Lewis acid or acceptor behavior is important for answering anions, and results from hydrogen bonding or from empty orbitals. Acceptor numbers and donor have been defined by measuring the strength of interaction between solvent molecules and the 'standard' and donor (OPCl3) acceptor (SbCl5) molecules, respectively. Values are shown in table and can give a useful guide although they ignore many specific details of the interaction and in particular make no distinction between 'soft' and 'hard' character. As like of this limitation, benzene is listed as having no
Table . Some ion-transfer solvents, with the characteristic solvent-system acid and base species, and other examples of acids and bases
appreciable donor strength, yet will dissolve silver perchlorate AgClO4 because of a rigid 'soft' donor-acceptor interaction between Ag+ and a benzene molecule.