Aqueous Metal Ions
As water can function as a Lewis base and transition metal ions can function as Lewis acids, most salts containing transition metals form coordination complexes when they are dissolved in water.
All salts which contain a first-row transition metal (i.e., Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn) form octahedral complex ions when they are dissolved in water.
In aqueous solution, transition metal cations are usually symbolized as Mn+(aq), where M is the atomic symbol of the metal ion and n is the charge on the ion. For example, Fe3+ in aqueous solution is written as Fe3+(aq). The (aq) symbol indicates that the metal ion is aquated.
Above is the octahedral complex produced when Fe(NO3)3 is dissolved in water. The nitrate counter ions are not shown. When the pH is above 3.5 the hydrolysis reaction will proceed with the splitting of water and ferric hydroxide is formed with the solution becoming more acidic. The FeO(OH) will precipitate out of solution. Hexaaquairon (III) and the subsequent ferric hydroxide are end products in the redox chemistry of pyrite, a major problem known as acid mine drainage. The precipitate referred to as “yellow-boy” is also commonly seen in iron petroleum pipelines.
Complex Formation In Aqueous Solution
When SCN- is added to an aqueous solution containing Fe(NO3)3, [Fe(OH2)6]3+ ions react with SCN- (not free Fe3+ ions).
The SCN- ion replaces a water molecule in the coordination sphere to produce the complex ion, [Fe(OH2)5(SCN)]2+. These two ions are hydration isomers, a subclass of coordination sphere isomers.