softness. This classification is an extension of the original theory of S. Ahrland, J. Chatt,
and N. R. Davies, who proposed that metal cations were classified in the order of the
stability constants Kf of the formation of the complexes of the metal cations with halide
anions. Namely, the order of Kf is I < Br < Cl < F toward metal ions belonging to class a ,and F < Cl < Br < I toward those of class b. The class a type metal cations are hard acids,
and class b type ones are soft acids. The metal cations which are not much dependent on the kind of halogens have borderline character.