Colors of Coordination Complexes
The color for a coordination complex can be predicted using the Crystal Field Theory (CFT). Knowing the color can have a number of useful applications, such as the creation of pigments for dyes in the textile industry. The reasoning behind why coordination complexes display such a great array of colors is merely coincidental; their absorption energy just happens to fall within range of the visible light spectrum. Chemists and physicists often study the color of a substance not to understand its sheer appearance, but more importantly for what the color describes. Color is an indicator of a chemical's physical proprieties on the atomic level.
The Electromagnetic Spectrum
The electromagnetic spectrum (EM) spectrum is made up of photons of different wavelengths. Photons, which are unique as they have properties of both waves and particles, enable us to see light and colors through a small portion of the EM spectrum. This visible light portion has wavelengths from roughly 400 to 700 nanometers (a nanometer, “nm,” is 10-9 meters). Each specific wavelength corresponds to a different color (see Figure 1), and when all the wavelengths are together we see “white light.”
The wavelength and frequency of a wave are inversely proportional--as one gets higher, the other gets lower (this is a consequence of all light traveling at the same speed). Due to this, blue light has a much higher frequency than red light, and consequently has more energy. In general, the farther you go to the right on the EM spectrum, the higher energy the photons have.
Color is perceived in two ways, through additive mixing, where different colors are made by combining different colors of light, and through subtractive mixing, where different wavelengths of light are taken out so that the light is no longer pure white. For colors of coordination complexes, we will be talking mainly about subtractive mixing. As shown in Figure 2, the basic idea behind subtractive mixing is this: white light (which is made from all the colors mixed together) hits an object, and then reacts with it. The object will absorb some of the light, and then reflect and/or (depending on the object) transmit the rest of the light, which goes to our eyes. We perceive the object as being whatever color is NOT absorbed. In Figure 2, white light (simplified by showing only green, red, and blue) is shown through a solution. The solution absorbs the red and green wavelengths, however the blue light is reflected and passes through, so we see the solution as being blue. Every time we see something as having color, this is the procedure that take place. If none of the light is absorbed, and all is reflected back off, we see the object as being white; if all of the light is absorbed, and there is none left to reflect or transmit through, we see the object as black.
Colors of Coordination Complexes--Crystal Field Splitting
When ligands attach to a transition metal to form a coordination complex, electrons in the d orbital split into high energy and low energy orbitals. The difference in energy of the two levels is referred to as ∆, which is a property both of the metal and the ligands (Figure 3). If ∆ is large, and a lot of energy is required to get electrons into the high energy orbitals, the electrons will instead pair up in the lower energy orbitals, resulting in a complex that is called “low spin” (Figure 4), however if ∆ is small, and it is simple to get electrons into the higher orbitals, the electrons will do so, and remain unpaired (until there are more than five electrons), resulting in a “high spin” complex (Figure 5). Different ligands are associated with either high or low spin--a ligand that is “strong field” will result in a large ∆ and a low spin configuration, while a ligand that is “weak field” will result in a small ∆ and a high spin configuration. For more details, see Crystal Field Theory (CFT).
Figure 3: d-Orbitals Splitting
Figure 4: Low Spin, Strong Field (∆o˃P) Figure 5: High Spin, Weak Field (∆o˂P)
The process of moving an electron to a higher energy orbital, and having it have a higher energy, means that it has the ability to absorb photons of higher energy. When this happens, and certain wavelengths are absorbed, subtractive color mixing takes place and the coordination complex solution becomes colored. If the ions have a noble gas configuration, and have no unpaired electrons, the solutions are seen as colorless (in reality, they still have a measured energy, and absorb certain wavelengths of light, however the wavelengths are not in the visible portion of the EM spectrum so we see no effect).
In general, the larger ∆, the higher energy of photons that is absorbed, and the farther to the right on the EM spectrum (Figure 1) the solution appears. This relationship is described in the equation ∆=hc/λ, where h and c are constants, and λ is the wavelength of light absorbed.
Using a color wheel can be helpful for determining what color a solution will appear based on what wavelengths it absorbs (Figure 6). If a complex absorbs a particular color, it will have the appearance of whatever color is across from it on the wheel. For example, if you determine that a complex absorbs photons in the orange range, you can conclude that the complex will look blue. You can also use this concept, and the equation stated above, to go backwards, and determine ∆ for a complex based on what color it is.
Relating the Colors of Coordination Complexes to the Spectrochemical Series
According to the Crystal Field Theory, ligands that have high spin are considered "weak field" and ligands that have low spin are considered "strong field." How does this relate to the colors we see in a coordination complex? High spin ligands absorb longer wavelengths (lesser frequencies) than do low spin ligands because the splitting energy is greater than the electron pairing energy, thus generating a small splitting. The energy difference, Δ, then determines the color of the coordination complex. According to the spectrochemical series, high spin ligands, which are considered "weak field," will absorb longer wavelengths of light.
Low spin ligands absorb wavelengths of greater frequency because the splitting energy is less than the electron pairing energy, thus generating a greater splitting. Since low spin ligands are considered "strong field," shorter wavelengths of light will be absorbed. The color that we see is the complementary color of the color absorbed. To predict which possible colors and their corresponding wavelengths are absorbed, we would use the spectrochemical series:
(Strong field/large Δ0) CO-, NO-, CN->NO2->en>py≈NH3>EDTA4->SCN->H2O>ONO->ox2->OH->F->SCN->Cl->Br->I- (weak field/ small Δ0)
If a solution appears yellow, what wavelength of light does it absorb? Would you expect it to be low spin or high spin?
A solution that looks yellow absorbs light that is violet, which is roughly 410 nm. Since it absorbs high energy, the electrons must be raised to a higher level, and ∆ must be high, so you would expect the complex to be low spin.
An octahedral metal complex absorbs light that is 535 nm. What is the crystal field splitting ∆ for the complex? What color is it?
You have two solutions, one that is orange, and another that is blue. You know that both solutions are made up of a cobalt complex, however one has chloride ions as ligands, while the other has ammonia ligands. Which solution would you expect to be orange?
We can see from this list that NH3 is a stronger ligand than Cl-, which means that the complex involving NH3 will have a greater ∆, and the complex will be low spin. Because of a larger ∆, the electrons will absorb higher energy photons, and the solution will have the appearance of a lower energy color. Since orange light is less energetic than blue light, we would expect the NH3 containing solution to be orange.
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