A structural model for Lewis acids and bases. An analysis of the structural chemistry of the acetate and trifluoroacetate ions

In the solid state acetate and trifluoroacetate ions show a variety of internal and external bonding geometries that can be directly related to the differences between the two anions and to differences in the Lewis-acid strengths of the counter ions. The structures and crystaline environments of 38 such anions have been examined. The results are interpreted in terms of a model of acid and base strength based on bond valences (bond strengths) determined from the observed bond lengths. In this model each atom is assigned a priori an acid or base strength equal to the valence of the bonds it normally forms. Acid–base bonds will only occur when the acid and base have similar strengths but some degree of mismatch can be accommodated by small but predictable deviations from the idealised structure. Thus when acetate and trifluoroacetate ions crystallise with weak Lewis acids (acid strength <0.5) the anions are symmetric and show little variation in their internal geometry but when they crystallise with strong Lewis acids the anions are often asymmetric and the methyl H atoms show enhanced acidity. The scales of acid and base strength proposed here account quantitatively for the observed variations in both the structure and chemical activity of the two anions examined.