Transition Metal Complexes and Color

Introduction

The d-orbitals of a free transition metal atom or ion are degenerate (all have the same energy.) However, when transition metals form coordination complexes, the d-orbitals of the metal interact with the electron cloud of the ligands in such a manner that the d-orbitals become non-degenerate (not all having the same energy.) The way in which the orbitals are split into different energy levels is dependent on the geometry of the complex. Crystal field theory can be used to predict the energies of the different d-orbitals, and how the d-electrons of a transition metal are distributed among them. When the d-level is not completely filled, it is possible to promote and electron from a lower energy d-orbital to a higher energy d-orbital by absorption of a photon of electromagnetic radiation having an appropriate energy. Electromagnetic radiations in the visible region of the spectrum often possess the appropriate energy for such transitions.

Seeing Color

The sensors in our eyes detect only those wavelengths in the visible portion of the electromagnetic spectrum.

Although visible light appears "white", it is made up of a series of colors. White light consists of three primary colors (red, yellow and blue). These primary colors can be mixed to make three secondary colors (orange, green and violet).


 Red  +  Yellow  makes  Orange 
 Yellow  +  Blue  makes  Green 
 Blue  +  Red  makes  Violet 



An "artist's" color wheel is a useful way show to these relationships. If you add the colors on opposite sides of the wheel together, white light is obtained. We only detect colors when one or more of the wavelengths in the visible spectrum have been absorbed, and thus removed, by interaction with some chemical species (see an animation of this here.) When the wavelengths of one or more colors is absorbed, it is the colors on the opposite side of the color wheel that are transmitted.




Example:

What happens when we see green?

 If red, yellow, orange, blue and violet are absorbed...

only one color is transmitted ...

GREEN
 If violet, red, and orange are absorbed...

blue, green , and yellow are transmitted ...

the middle color is perceived...

GREEN



Grass and leaves appear green because chlorophyll absorbs wavelengths in the red and blue portion of the visible spectrum. The wavelengths in between (green) are transmitted.

Transition Metal Complexes

When light passes through a solution containing transition metal complexes, we see those wavelengths of light that are transmitted. The solutions of most octahedral Cu (II) complexes are blue. The visible spectrum for an aqueous solution of Cu (II), [Cu(H2O6]2+, shows that the absorption band spans the red-orange-yellow portion of the spectrum and green, blue and violet are transmitted.

The absorption band corresponds to the energy required to excite an electron from the t2g level to the eg level.

Recall, the energy possessed by a light wave is inversely proportional to its wavelength. The Cu(II) solution transmits relatively high energy waves and absorbs the low energy wavelengths. This indicates that the band gap between the two levels is relatively small for this ion in aqueous solution.

d-Orbital Splitting

The magnitude of the splitting of the d-orbitals in a transition metal complex depends on three things: The Nature of the Ligands
Some ligands only produce a small energy separation among the d-orbitals while others cause a wider band gap. Ligands that cause a small separation are called weak field ligands, and those that cause a large separation are called strong field ligands. The ordering of their splitting ability is called the spectrochemical series.here.

A comparison of the visible absorption maxima for a number of cobalt (III) complexes shows the effects of ligands on the d-orbital band gap.





References:
Kotz, J.C.; Purcell, K.F. Chemical and Chemical ReactivitySaunders: New York, 1987, Chapter 25.
Rodgers, G.E. Introduction to Coordination, Solid State, and Descriptive Inorganic ChemistryMcGraw -Hill: New York, 1994, Chapter 4.