A classification of metal ions according to the stability of their complexes for a given ligand. Type A metals cations include the ions of group 1 (Li+ to Cs+), the ions of group 2 (Be2+ to Ba2+), and ions of lighter transition metals in high oxidation states (e.g. Co3+, Ti4+, Fe3+). The type B metal cations are those of heavier transition metals in lower oxidation states (e.g. Ag+, Cu+, Ni2+, Pd2+, Pt2+). Certain ligands tend to form more stable complexes with type A metals; others form more stable complexes with type B. For example, the tendency of halide anions to complex with type A metals is in the sequence F− > Cl− > Br− > I−. Their tendency to complex with type B metals is the opposite sequence. This led to a classification of ligands into type A ligands (e.g. F−) which tend to complex with type A metals and type B ligands (e.g. I−), which tend to complex with type B metals. The classification was introduced by Ahrland in 1958. See also HSAB principle.
F− > Cl− > Br− > I−.