• We shall first cover the definition of transition metal ions in aqueous solution.
• We'll look at how water acts as a ligand for transition metal ions.
• We'll then cover the formation of complex ions and look at the splitting of the d orbitals by ligands.
• Finally, we shall discover why complex ions are coloured in solutions.

Transition Metal Ions in Aqueous Solution

First, we shall discuss the nature of metal ions in solution and, more importantly, how they form complex ions.

Transition metals are metals in the d block of the periodic table. They are defined by their irregular electron configurations. Their high charge density when in their ionic form coupled with their small size gives them interesting properties in water.

They can form complex ions in solution, due to the fact that water can act as a ligand. Transition metal ions will always form complex ions when in solution with ligands. A ligand can be any molecule or solvent that can donate a lone pair of electrons, including bases and nucleophiles. Ligands are able to stabilise the metal ion in water through this interaction of lone pair bonding. This stabilisation needs to occur to stabilise the positive charge in an aqueous environment.

Here we shall specifically discuss transition metal ions in aqueous solutions and see how they behave in water-based environments.

There are three key parts of a complex ion to identify. These are the transition metal, the ligands, and the geometry of the complex ion.

You will notice that ligands always surround the transition metal ion in a particular geometric way. In aqueous environments, water will usually surround a transition metal ion with an octahedral geometry, as there will be six substituents around the central metal ion.

Here is a schematic of six water molecules surrounding a transition metal ion in aqueous solution.

Fig. 1 - Diagram of water ligands surrounding a central metal ion in an octahedral geometry

Do you notice how the overall charge of the ion is not just at the transition metal central ion, but is distributed throughout the whole complex ion? It is written outside the brackets to signify that the ligands take on some of the charge and stabilise it.

Other Types of Complex Ions

Other transition metal ion complexes can arise which have a different geometry and charge. Here are some common examples:

[M(H₂O)₆]²⁺, where M can only be Fe and Cu

[M(H₂O)₆]³⁺, where M can only be Fe and Al

A key thing to remember is that the acidity of metal ions will change based on the charge. The acidity of [M(H₂O)₆]³⁺ is greater than that of [M(H₂O)₆]²⁺. This can be attributed to the charge and size ratios of the transition metal ion, which ultimately determine the physical properties of the complex ion formed.

Additionally, some metal hydroxides show amphoteric character by dissolving in both acids and bases. For example, the metal ion of aluminium Al³⁺.

Example of a Transition Metal Ion in Aqueous Solution

Below you will find a common example of a complex ion. Copper sulphate forms a light blue solution in water. This is due to the copper ions with a charge of 2+ being chelated by the ligand, in this case water, to produce a complex ion.

Fig. 2 - Copper ion in aqueous solution

Stability of Transition Metal Ions in Aqueous Solutions

Due to the fact that the transition metal ion is very small in its ionic radius but holds such a high charge, it is necessary to thermodynamically stabilise this force within the system, which in this case is the aqueous environment.

Ligands stabilise the transition metal ion in many ways, allowing the transition metal ion to be present in aqueous solutions. First of all, the ligands take on the positive charge and distribute it amongst themselves.

The charge is shared within the whole complex ion, thus stabilising it in an aqueous solution.

Secondly, ligands are able to stabilise the orbitals of the transition metal ion. This is a key property that is crucial to elements within the 3d block. The orbitals in transition metals require stabilisation due to their electronic arrangement, as the orbitals in the d block are at the same energy level.

Upon dative covalent bonding with ligands, the d orbitals of the transition metal are split into two energy levels. The diagram below shows the split of orbitals.

Fig. 3 - Splitting of the d orbital by ligands

This allows for the direct stabilisation of the orbitals of the transition metals through the ligands. The five orbitals are split into two higher energy state ones and three lower energy state ones. This is the case for octahedral complexes. This results in a stable octahedral configuration of the ligands in solution to produce a stable transition metal complex ion in an aqueous environment.

The splitting of the d orbitals is able to not only stabilise the orbitals and thermodynamically provide a justification for the orbitals in an aqueous environment, but also give very interesting properties to the complex ion that will be discussed in the next section.

Colours of Transition Metal Ions in Aqueous Solutions

Transition metal ions in water absorb specific frequencies of the visible light spectrum, but why is this? When the d orbitals are split from the ligands, they are split into two higher energy orbitals and three lower energy orbitals. This arbitrary shift causes a separation of the energy levels to be equivalent to a specific wavelength.

On the diagram in the previous section above, you will notice that the splitting of orbitals is denoted with a delta E, signifying a change in energy. This energy gap corresponds to a specific wavelength of light.

When an electron is excited through electromagnetic radiation, it will go to a higher energy level. In the case of transition metal ions in aqueous solutions, this will correspond to an electron jumping from the lower three orbitals to the higher two orbitals of the split d orbitals. This phenomenon occurs only in the d block elements since they have a partially filled d orbital.

When an electron jumps from the lower to the higher energy levels of the split d orbitals, it absorbs the specific wavelength of light that the energy gap corresponds to. This means that if you shine white light at the complex ion, one specific wavelength will be absorbed and the rest will be reflected. Thus, to us, the complex will show the complementary colour on the colour wheel.

Transition Metal Ions in Aqueous Solutions

Here you will find a diagram of how a complex ion is formed. You can use this schematic diagram to outline how transition metal ions are bonded through dative covalent bonds with lone pairs of electrons of water molecules in aqueous solutions, ultimately becoming stabilised.

Fig. 4 - Diagram of formation of transition metal ions in aqueous solutions