Color



 

Learning Objective

  • Discuss the process which provides color in coordination complexes.

Key Points

    • The colors in metal complexes come from the d orbitals because they are not involved in bonding.
    • d-d electron transitions are allowed in complexes if the center of symmetry is disrupted, resulting in a vibronic transition.
    • In Metal-to-Ligand Charge Transfer (MLCT), electrons can be promoted from a metal-based orbital into an empty ligand-based orbital.
    • An electron may jump from a predominantly ligand orbital to a predominantly metal orbital (Ligand-to-Metal Charge Transfer or LMCT).
    • Coordination complex color results from the absorption of complimentary colors.

Terms

  • ligandAn ion, molecule, or functional group that binds to another chemical entity to form a larger complex.
  • band theoryIn a solid, those ranges of energy that an electron is allowed to have.
  • centrosymmetricHaving a center of symmetry.
  • orbitalA specification of the energy and probability density of an electron at any point in an atom or molecule.

Color in Coordination Compounds

Metal complexes often have spectacular colors caused by electronic transitions induced by the absorption of light. For this reason, they are often applied as pigments. We know that light can be emitted corresponding to the difference in energy levels. We could expect them to come from the d-orbitals. This is because they are not involved in bonding, since they do not overlap with the s and p orbitals of the ligands. Most transitions that are related to colored metal complexes are either d–d transitions or charge band transfer.

d-d Transitions

In a d–d transition, an electron in a d orbital on the metal is excited by a photon to another d orbital of higher energy. In complexes of the transition metals, the d orbitals do not all have the same energy. In centrosymmetric complexes, d-d transitions are forbidden by the Laporte rule. The Laporte rule states that, if a molecule is centrosymmetric, transitions within a given set of p or d orbitals are forbidden. However, forbidden transitions are allowed if the center of symmetry is disrupted. Transitions that occur as a result of an asymmetrical vibration of a molecule are called vibronic transitions. Through such asymmetric vibrations, transitions that would theoretically be forbidden, such as a d-d transition, are weakly allowed.

An example occurs in octahedral complexes such as in complexes of manganese(II). It has a d5 configuration in which all five electrons have parallel spins. The color of such complexes is much weaker than in complexes with spin-allowed transitions. In fact, many compounds of manganese(II), like manganese(II) chloride , appear almost colorless. Tetrahedral complexes have somewhat more intense color. This is because mixing d and p orbitals is possible when there is no center of symmetry. Therefore, transitions are not pure d-d transitions.

Example of weaker color due to d-d transitionSample of manganese(II) chloride.

Change Band Transfer

Electrons can also be transferred between the orbitals of the metal and the ligands. In Metal-to-Ligand Charge Transfer (MLCT), electrons can be promoted from a metal-based orbital into an empty ligand-based orbital. These are most likely to occur when the metal is in a low oxidation state and the ligand is easily reduced. Ligands that are easily reduced include 2,2′-bipyridine (bipy), 1,10-phenanthroline (phen), CO, CN-, and SCN-. An example of color due to MLCT is tris(2,2′-bipyridyl)ruthenium(II), which is a versatile photochemical redox reagent.

Example of color due to MLCT transitionSample of tris(bipyridine)ruthenium(II)-chloride

Conversely, an electron may jump from a predominantly ligand orbital to a predominantly metal orbital (Ligand-to-Metal Charge Transfer or LMCT). These can most easily occur when the metal is in a high oxidation state. For example, the color of chromate, dichromate, and permanganate ions is due to LMCT transitions.

Examples of color due to LCMT transitionsSamples of (from top to bottom) potassium chromate, potassium dichromate, and potassium permanganate.

“Seeing” Color

We can perceive colors for two reasons: either we see it because that color is the only color not absorbed or because all colors of visible light are absorbed except for a particular color known as its complimentary color.

Large energy differences should correspond to smaller wavelengths and purple colors, while small energy differences should result in large wavelengths and colors closer to red. For example, you might expect to see red for a complex with a small energy gap and large wavelength. Green is the compliment of red, so complexes with a small energy gap will actually appear green.

The color we see for coordination complexes is a result of absorption of complimentary colors. A decrease in the wavelength of the complimentary color indicates the energy gap is increasing and can be used to make general rankings in the strengths of electric fields given off by ligands. These phenomena can be observed with the aid of electronic spectroscopy (also known as UV-Vis).