Most aquo complexes are mono-nuclear, with the general formula , with  or 3; they have an octahedral structure. The water molecules function as Lewis bases, donating a pair of electrons to the metal ion and forming a dative covalent bond with it. Typical examples are listed in the following table. 
Tutton's salts are crystalline compounds with the generic formula  (where , , , , , or ). Alums, , are also double salts. Both sets of salts contain hexa-aquo metal cations.