A method of classifying Lewis acids and bases (See acid) developed by Ralph Pearson in the 1960s. The acronym stands for `hard and soft acids and bases'. It is based in empirical measurements of stability of compounds with certain ligands. Hard acids tend to complex with halide ions in the order F− > Cl− > Br− > I− Soft acids complex in the opposite order. Compounds that complex with hard acids are hard bases; ones that more readily form complexes with soft acids are called soft bases. In general, soft acids and bases are more easily polarized than hard acids and bases and consequently have more covalent character in the bond. The idea is an extension of the type A and B metals concept to compounds other than metal complexes.
F− > Cl− > Br− > I−