25.3Complex Ions

25.3 Complex Ions

Ligand and complex

A ligand is a molecule or ion with a lone pair of electrons, capable of forming a co-ordinate bond to a metal atom or ion. Some examples of ligands are water, ammonia and chloride ion
A complex is a molecule or ion containing a central metal ion surrounded by one or more ligands. Some examples are [Al(H2O)6]³⁺ and [Fe(H2O)6]³⁺.

Formation of a complex

A complex is formed when ligands bond with the central metal ion via co-ordinate bonding.

Take [Fe(H2O)6]³⁺ as an example:
1. Fe ion has an electronic structure of [Ar]3d⁶4s². When it loses electrons to become Fe³⁺, the electronic structure becomes [Ar]3d⁵.
2. The ion then uses 6 orbitals from the 4s, 4p and 4d levels to accept lone pairs from the water molecules

Note: i. For small ligands such as water and ammonia, six is the maximum number of ligands it can bond to the central metal
ii. For larger ligands such as chloride ions, it can only form four co-ordinate bonds with the central metal ion.

The co-ordination number of a complex is the total number of co-ordinate bond the ligands formed with the central metal ion. For example, [Fe(H2O)6]³⁺ has a co-ordination number of 6

Some other co-ordination numbers and the geometry of the complex:

A ligand is said to be unidentate if it forms one co-ordinate bond with the central metal ion Water, ammonia and chloride are unidentate ligands.
1. A ligand is said to be bidentate if it forms two co-ordinate bonds with the central metal ion. 1,2-diaminoethane and ethandioate ion are bidentate ligands.
2. A complex ion which has bidentate ligands attached to it exhibits optical isomerism.
3. In addition, a complex ion which has the ligands arranged in a square planar structure exhibits geometrical isomerism.

Naming complexes

Naming complexes is very similar in naming organic compounds

Each ligand is given a code:

Some examples:
1. [Fe(H2O)6]³⁺ is hexaaquairon(III) ion
  - hexa shows there are six ligands
  - aqua shows the ligands are water molecules
  - iron shows the name of the central metal ion
  - (III) shows the the oxidation number of the central metal ion

If there are more than one type of ligands, the ligands are named according to alphabetical order, ignoring the prefixes. For example, [Cu(NH3)4(H2O)2]²⁺ is called tetraamminediaquacopper(II) ion
1. For cationic complex, the name of the central metal ion is the same as its ooo original name.
2. For anionic complex, the name of the central metal ion follows this coding:
3. For example, [CuCl4]²⁻ is called tetrachlorocuprate(II) ion. (It is yellow colour)

Ligand exchange

A ligand exchange reaction is a reaction in which one ligand in a complex ion is replaced by a different one.
Example 1: Replacing water with chloride ions.
1. When concentrated hydrochloric acid is added to a solution containing hexaaquacobalt(II) ions, the solution turns from pink to dark rich The six water molecules are replaced by four chloride ions.
2. Only four chloride ions are attached because chloride ions are larger, therefore six chloride ions cannot fit in around the central metal ion
3. The co-ordination number of the complex changes from 6 to 4. The geometry of ligands around changes from octahedral to tetrahedral
4. By Le Chatelier’s principle, the process can be reversed by adding water to it
Example 2: Replacing water with ammonia.
1. The ammonia can act as a base and also a ligand. This is because hexaaqua- ions are acidic.
2. The acidity of hexaaqua- ions is due to the electron pair in the water molecule being pulled towards the central positive charge, making the hydrogen more positive, and easily lostStage 1: Ammonia as a base(Acid-base reaction)
3. When small amount of ammonia is added, hydrogen ions from the water are pulled out, leaving hydroxide ions
4. A precipitate will be formed when two hydrogen ions have been This is because a neutral complex is produced and it is unattractive to water molecules. Hence, it is no longer soluble in water.Stage 2: Ammonia as a ligand(ligand exchange reaction)
5. When excess ammonia is added, the precipitate dissolves. The ammonia replaces water as a ligand to give tetraamminediaquacopper(II) ions. Notice that only 4 of the 6 water molecules are replacedOverall reaction:

Certain ligands are stronger than other and central metal ions generally prefer to be bonded to stronger ligands. Therefore a ligand exchange reaction may not happen if the new ligand is weaker than the original ligand

Colour of complexes

Visible light is the visible part of the electromagnetic spectrum which has wavelengths between 400 nm to 700 nm

White light is composed of all the wavelengths in the visible part of the electromagnetic When white light is passed through a prism, it splits into its constituent colours. This phenomenon is known as dispersion of light.
When white light passes through copper(II) sulfate, CuSO4 solution, visible light with wavelengths in the red and yellow region is absorbed. The unabsorbed ones combine to given a pale blue(cyan) colour

Shapes of d-orbitals

There are five types of d-orbitals, named dxy, dxz, dyz, dz² and dx²-y².
1. dxy, dxz and dyz have the ‘lobes’ in the x-y, x-z and y-z planes respectively. Note that the ‘lobes’ does not touch the axes, they are located between the axes.
2. dz² has the ‘lobes’ along the z-axis, with a ‘collar’.
3. dx²-y² has the ‘lobes’ along the x- and y-axes.

Factors affecting the colour of complexes

The colour of a complex ion depends on the wavelength(or frequency) of the visible light absorbed. This in turn depends on the energy gap, ∆E between the two groups of d-orbitals(see below).

For an octahedral complex:
1. The central metal ion has five degenerate d-orbitals. They split into two groups, two with higher energy and the other three with lower when ligands are attached to it
2. This is because as the lone pairs on the ligands approach the central metal ion, the d-electrons in the central metal ion are repelled.
3. Due to the shape of the octahedral complex, the d-electrons repelled are the ones in the dz² and dx²-y² orbitals, and these orbitals will have higher energy

Other shapes have the splitting as below(not required in A-levels).

The size of the energy gap, ∆E depends on:
i. the oxidation number of the central metal ion.
– In general, the higher the oxidation state of the central metal ion, the larger the size of ∆E.ii. the nature of ligands.
– In general, the splitting, ∆E is larger if the lone pair comes from a less electronegative atom

The origin of colour of complexes:
1. As the electromagnetic radiation(photons) with frequencies within the visible region of the electromagnetic spectrum passes through the complex, some of the photons are absorbed
2. The photon absorbed must have energy equal to the energy gap of the two groups of d-orbitals, which is given by ∆E = hf.
3. The absorbed energy is used to promote an electron from a lower energy d-orbital to a higher energy one. This is called d-d transition.
4. The colour of the complex is the result of the unabsorbed frequencies of light It can then be estimated using the complementary colour wheel.

The reason why different complexes have different colours is because they all have different energy gap size, ∆E. The incoming photon must have frequency exactly equal to ∆E/h to be absorbed and be used to promote an electron. Different colours and wavelengths of light have different frequencies, which is why they have different colours

Non-transition metal complexes are often not coloured because they do not have partly-ftlled d-electrons. Hence no photon is absorbed and no electron is promoted

Note: for non-physics students:
1. The higher the frequency, f of light(photon), the higher its energy, which is given by ∆E = hf, where h is the Planck’s constant
2. There is an inverse relationship between frequency, f and wavelength, λ, which is given by c = fλ, where c is the speed of light.
3. In the visible spectrum, red light has the lowest energy and violet the highest