The salts in which ion or neutral molecules form a co-connective bond with a metallic cation are called complex salts.
Example:
K4[Fe(CN)6] and [Cu(NH3)4]Cl2 are complex salts.
K4[Fe(CN)6] has a sub-valence bond between Fe2+ and CN– and [Cu(NH3)4]Cl2 has a sub-valence bond between Cu2+ and NH3.
Complex ions are obtained by dissolving complex salts in water. Complex ions in solutions do not dissociate in their components or have very small amounts of ionization.