Complex ions formed by the elements Cr, Mn, Fe, Co, Ni and Cu with the ligands H2O, NH3 and Cl–
• OH-acts as a ligand in few d block cations.
eg. [Fe(H2O)5OH]2+ formed by the hydrolysis of hydrated Fe3+ ions. However, most
metal ions with NaOH or NH3 give insoluble hydroxides and not hydroxo complexes.
eg. Cr(OH)3 – Green, Fe(OH)3 – Reddish brown, Fe(OH)2 – Dirty green, Cu(OH)2 – Blue, Mn(OH)2 – White
• The colour of the complex varies depending on the central metal atom.
eg. [Cr(H2O)6]3+ – Blue – violet
[Fe(H2O)6]3+ – Yellow
• The colour of the complex varies depending on the oxidation state of the central metal atom.
eg. [Fe(H2O)6]2+ , Pale green
[Fe(H2O)6]3+ – Yellow
[Mn(H2O)6]2+ – Pale pink
[Mn(H2O)6]3+ – Violet
• The colour changes also when the ligands change.
eg. [Co(H2O)6]2+ – Pink
[Co(NH3)6]2+ – Yellow brown
