M³⁺ Aqua ions (M = Al, Ga, In, Tl)

   TheM³⁺‏ aqua ions (M = Al, Ga, In, Tl) are acidic and the acidity increases down the group. Solutions of their salts are appreciably hydrolysed and salts of weak acids (e.g. carbonates and cyanides) cannot exist in aqueous solution. Solution NMR spectroscopic studies show that in acidic media, Al(III) is present as octahedral [Al(H2O)₆]³⁺‏, but raising the pH leads to the formation of polymeric species such as hydrated [Al₂(OH)₂]^4‏+ and[Al₇(OH)₁₆]⁵⁺‏. Further increase in pH causes Al(OH)₃ to precipitate, and in alkaline solution, the aluminate anions [Al(OH)₄]^-  (tetrahedral) and [Al)OH(₆]^3- (octahedral) and polymeric species such as [(HO)₃Al(µ-O)Al(OH)₃]^2- arepresent. The aqueous solution chemistry of Ga(III) resembles that of Al(III), but the later metals are not amphoteric.