For cations build from metals in early groups in the periodic table the complexing strength with halide ions follows the sequence
whereas with many post-transition metals and some later transition metals the reverse sequence is found (e.g. Pt2+, Hg2 +, Pb2+;). The former behavior is known as the latter as class b and class a behavior, and the difference is an example of soft and hard properties, respectively. Class b ions form rigid complexes with ligands such as ammonia, which are lighter than water, whereas class a ions do not complex with such ligands appreciably in water.