text: Solutions of metal aquo complexes are acidic owing to the ionization of protons from the water ligands. In dilute solution chromium(III) aquo complex has a pK of about 4.3:
  Thus, the aquo ion is a weak acid, of comparable strength to acetic acid (pK of about 4.8). This pK is typical of the trivalent ions. The influence of the electronic configuration on acidity is shown by the fact that  () is more acidic than  (), despite the fact that Rh(III) is expected to be more electronegative. This effect is related to the stabilization of the pi-donor hydroxide ligand by the (t) Ru(III) centre.
  In concentrated solutions, some metal hydroxo complexes undergo condensation reactions, known as olation, to form polymeric species. Many minerals are assumed to form via olation. Aquo ions of divalent metal ions are less acidic than those of trivalent cations.
  The hydrolyzed species often exhibit very different properties from the precursor hexaaquo complex. For example, water exchange in  is 20000 times faster than in .