A classification of metal ions according to the stability of their complexes for a given ligand. Type A metal 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

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 Sten Ahrland in 1958. See also HSAB principle.