The H2O exchange rate in aqua ions Inert, intermediate, and labile are classification of the exchange rate proposed by H. Taube (1952).The exchange rate of aqua ions (ionscoordinated by water molecules) of main-group and transition metals differ greatly depending upon the identity of the metal species. Since the rate of water ligand exchange is well correlated with the exchange rates of other ligands, it is useful for general comparision of the exchange rates in the complexes of different metal ions. For alkali and alkaline earth metals, the exchange rates are very high (105-109 s-1), and the complexes of these metals are classified as labile. As the dissociative mechanism is generally found in these cases, ions with smaller ionicity and of larger size attract water ligands less and their exchange rates becomes higher. In Group 12 metal ions Zn2+, Cd2+, Hg2+, Group 13 metal ions Al3+,Ga3+ In3+ and Group 3 metal ions Sc3+, Y3+, rapid water ligand exchange takes place by a dissociative mechanism.