The class (b) ions form halides whose solubility is in the order F- > Cl- > Br- > I-. The solubility of Class (a) halide is in the reverse order. The calss (b) metals ions also have a larger enthalpy of reaction with P donor than with N donor, again the reverse order of the Class (a) metal ion recations.Class (b) – having d electrons available for π bonding Tl(III) show stronger Class (b) character than Tl(I) because Tl(I) has two 6s electrons that screen the 5d electrons and keep them form being fully available for π bonding