Colorful formation of complex ions

A light pink solution turns blue, and then turns pinkish-blue ... lavender.

Ingredients: cobalt chloride, hydrochloric acid, water

Procedure: A partial recipe follows.

1. Prepare a solution of cobalt chloride in 98% ethyl alcohol.

2. Add hydrochloric acid to the solution and observe change in color.

3. Add water to the solution and observe transformation in color of solution.

Understanding: A solution of cobalt(II)chloride is a lovely pink color. The color is due to the presence of the pink hexaaqua cobalt(II) complex ion.

Co2+ + 6 H2O → [Co(H2O)6]2+

The addition of hydrochloric acid to the solution provides a high concentration of chloride ion. The addition of chloride ion drives the formation of the tetrachloro cobalt(II) complex ion.

[Co(H2O)6]2+ + 4 Cl- → [CoCl4]2- + 6 H2O

Using our understanding of LeChatelier's Principle, we can push this equilibrium to the right, by the addition of more reactant, or the left, by the addition of more product.

The addition of chloride ion pushes the equilibrium to the right favoring the formation of the blue tetrachloro cobalt species product. Addition of water to the solution pushes the equilibrium to the left favoring the pink hexaaqua cobalt species reactant.

If we add a sufficient amount of water to the blue solution dominated by the blue tetrachloro cobalt(II) complex ion, we can convert some of the tetrachloro cobalt(II) complex ion to hexaaqua cobalt(II) complex ion. The result is a solution with both pink and blue complex ions present, and a lavender color.

The quantum mechanics of complex ion formation

We have used quantum mechanical models to study the one electron atom, multielectron atoms, diatomic molecules, and polyatomic molecules. In each case, we have determined the discrete allowed energies of the system, and the one-electron wave functions or orbitals corresponding to each of the allowed energies. Using the Aufbau Principle, Pauli Principle, and Hund's Rule, we have were able to build the lowest energy ground state electron configurations for those systems. Through the electron configuration, we are able to develop a fundamental quantum mechanical understanding of the structure, ionization energy, bond strengths, magnetism, and spectroscopic properties of the atom or molecule. Quite powerful!

We would like to develop the same level of understanding of coordination compounds. We will find that a few simple rules provide us with a powerful means of understanding the detailed structure, magnetism, and spectroscopic properties of transition metal complexes.

The place to start is the central transition metal ion. We can determine the electron configuration of the ion alone. We can apply our rule of thumb that the ions of the first row transition metals have no 4s electrons. That leads to an electron configuration for Co2+ of

[Ar] 3d7

For the isolated ion, the five 3d-orbitals will have identical energies. However, when the ion is surrounded by the coordinating ligands, things change! In the case of the hexaaquacobalt(II) complex ion, the central cobalt ion is surrounded by six coordinating water molecules. Appealing to the rules of Valence Shell Electron Pair Repulsion Theory, we expect the water molecules to be arranged in an octahedral geometry. And they are!

The interaction of the water molecules with the cobalt ion can be understood using a localized electron model. In that model, a lone pair of electrons on the water molecule are donated to an atomic orbital on the cobalt ion. As such, the cobalt ion acts as a Lewis acid and the water molecule as a Lewis base. The cobalt ion forms six equivalent

sp3d2

hybrid atomic orbitals to accept the electron density donated by the coordinating water molecules.

The interaction between the coordinating ligands and the central metal ion can be understood in terms of two competing interactions. First, there is the attractive interaction of the charge and/or dipole moment of the coordinating ligand and the metal ion. That favorable interaction lowers the energies of the 3d orbitals. Second, there is the repulsive interaction of the donated electron pairs of the ligands and the electrons of the metal ion.

Complex ion formation and the spectrochemical series

Each surrounding ligand field will have a special geometry. In the case of the hexaaquacobalt(II) complex, the water molecules form an octahedral ligand field. The repulsive energy of interaction between the electrons in the water molecules will be different for different d-orbitals. For example, electrons in the atomic dz2 orbital will repel localized electrons on the coordinating waters positioned along the positive and negative z-axis of the octahedral ligand field. That repulsive interaction will raise the energy of the atomic dz2 orbital above that of the dxy, dyz, and dxz orbitals. The dx2-y2 orbital is similarly repelled by the octahedral ligand field. The repulsive interactions split the energies of the five 3d-orbitals on the cobalt ion into two groups, the lower energy dxy, dyz, and dxz orbitals and the higher energy dx2-y2 and dz2 orbitals. The difference in energy between the two sets of d-orbitals is known as the ligand field splitting, &Deltao, for the octahedral complex

dx2-y2 dz2

Δo                                

dxy dxz dyz

That relative ordering of the d-orbitals is dictated by the geometry of the octahedral ligand field and will be the same for any octahedral ligand field, whether it is created by water ligands, chloride ligands, or ammonia ligands. The magnitude of the ligand field splitting, Δo, will be determined by the nature of the coordinating ligands. The ligands that make the most intense interactions, leading to the largest values of Δo, are known as strong field ligands. Examples include the ammonia molecule and the cyanide ion. Ligands that make a relatively weak interaction, resulting in a small ligand field splitting, are known as weak field ligands. Examples include fluoride and chloride ions.

The ordering of ligands from weak field to strong field is

Cl- < F- < H2O < NH3 < CN-

weak field                     strong field

This ordering is known as the spectrochemical series. The nature of the ligand will determine the magnitude of the ligand field splitting, Δo. The magnitude of &Deltao will determine the energy of transition when the valence electrons of the metal ion absorb a photon of light. The color of the absorbed light will be determined by the energy of the photon

Δo = hc/λ

and the apparent color of the solution will be determined by the color of the transmitted light.

The magnetic properties of coordination compounds

We are almost there. We understand the shape of the complex ion, and the ordering of the 3d-orbitals on the cobalt ion. Now we need to build the electron configuration of the cobalt ion. We follow the Aufbau Principle, Pauli Principle and Hund's Rule and file six electrons first into the lower dxy, dyz, and dxz orbitals. We have one electron remaining and that can go into either of the higher lying dx2-y2 and dz2 orbitals, leading to an electron configuration

dxy2 dxz2 dyz2 dz21

This electron configuration has the most paired electrons possible and is known as the low spin arrangement. But hold on! There is another possibility. Suppose that the splitting in energy between the lower and higher lying d-orbitals is small compared with the electron-electron repulsion in a given 3d-orbital. Hund's Rule would have us first half-fill the five 3d-orbitals before pairing electrons, leading to the electron configuration

dxy2 dxz2 dyz1 dx2-y21 dz21

This electron configuration has the most unpaired electrons possible and is known as a high spin arrangement.

Let's look at a few examples. If the cobalt(III) ion is surrounded by weak field chloride ligands, as in CoCl63-, we find that the ligand field splitting is small compared with the electron-electron repulsion. The electron configuration of cobalt will be found in a high spin arrangement. If the cobalt(III) ion is surrounded by strong field ammonia ligands, as in Co(NH3)63+, we find that the ligand field splitting is large compared with the electron-electron repulsion. The electron configuration of cobalt will be found in a low spin arrangement.

What about the cobalt(II) ion surrounded by an octahedral ligand field of water? Water is an intermediate field ligand. The cobalt(II) ion is found to have a low spin arrangement.

The quantum mechanics of a coordination complex

Question: Consider the tetrachlorocobalt(II) complex ion. What is the electron configuration of the cobalt ion? What is the hybridization of the atomic orbitals on the central cobalt ion? Are the ligands that coordinate the central cobalt ion strong or weak field ligands? Is the cobalt in a high or low spin state? Is the compound paramagnetic or diamagnetic?

You can check your answers here.