For example, if a complex is known to absorb photons in the orange range, it can be concluded that the solution will look blue. This concept can be used in reverse to determine ∆ for a complex from the color of its solution. Relating the Colors of Coordination Complexes to the Spectrochemical Series According to the Crystal Field Theory, ligands that have smaller \(\Delta\)) values are considered "weak field" and will absorb lower-energy light with longer \(\lambda\) values (ie a "red shift"). Ligands that have larger \(\Delta\)) values are considered "strong field" and will absorb higher-energy light with shorter \(\lambda\) values (ie a "blue shift"). This relates to the colors seen in a coordination complex. Weaker-field ligands induce the absorption of longer wavelength (lower frequency=lower energy) light than stronger-field ligands since their respective \(\Delta_o\) values are smaller than the electron pairing energy. The energy difference, \(\Delta_o\), determines the color of the coordination complex. Photons, unique in displaying the properties of both waves and particles, create visible light and colors in a small portion of the EM spectrum. This visible light portion has wavelengths in approximately the 400-700 nanometer range (a nanometer, “nm,” is 10 -9 meters). Each specific wavelength corresponds to a different color (Figure (Public Domain; Gringer via Wikipedia) The wavelength and frequency of a wave are inversely proportional: as one increases, the other decreases; this is a consequence of all light traveling at the same speed. \[\lambda \propto \nu^{-1} \nonumber \] Because of this relationship, blue light has a much higher frequency and more energy than red light. Perceiving Color 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, subtractive mixing is considered. As shown in Figure \(\PageIndex{2}\), the idea behind subtractive mixing is that white light (which is made from all the colors mixed together) interacts with an object. The object absorbs some of the light, and then reflects or transmits (or both, depending on the object) the rest of the light, which contacts the eye. The object is perceived as whichever color is not absorbed. In Figure \(\PageIndex{2}\), white light (simplified as green, red, and blue bands) is shone through a solution. The solution absorbs the red and green wavelengths; however, the blue light is reflected and passes through, so the solution appears blue. As certain wavelengths are absorbed in this process, subtractive color mixing occurs and the coordination complex solution becomes colored. If the ions have a noble gas configuration, and have no unpaired electrons, the solutions appear colorless; in reality, they still have a measured energy and absorb certain wavelengths of light, but these wavelengths are not in the visible portion of the EM spectrum and no color is perceived by the eye. In general, a larger \(∆_o\) indicates that higher energy photons are absorbed, and the solution appears further to the left on the EM spectrum shown in Figure \(\PageIndex{1}\). This relationship is described in the equation \[∆_o=hc/λ \nonumber \] where \(h\) and \(c\) are constants, and \(λ\) is the wavelength of light absorbed. Using a color wheel can be useful for determining what color a solution will appear based on what wavelengths it absorbs (Figure \(\PageIndex{6}\)). If a complex absorbs a particular color, it will have the appearance of whatever color is directly opposite it on the wheel. For example, if a complex is known to absorb photons in the orange range, it can be concluded that the solution will look blue. This concept can be used in reverse to determine ∆ for a complex from the color of its solution. Figure \(\PageIndex{5}\): Color wheel with wavelengths marked (CC BY-SA 4.0 International; Tem5psu via source ) Relating the Colors of Coordination Complexes to the Spectrochemical Series According to the Crystal Field Theory, ligands that have smaller \(\Delta\)) values are considered "weak field" and will absorb lower-energy light with longer \(\lambda\) values (ie a "red shift"). Ligands that have larger \(\Delta\)) values are considered "strong field" and will absorb higher-energy light with shorter \(\lambda\) values (ie a "blue shift"). This relates to the colors seen in a coordination complex. Weaker-field ligands induce the absorption of longer wavelength (lower frequency=lower energy) light than stronger-field ligands since their respective \(\Delta_o\) values are smaller than the electron pairing energy. The energy difference, \(\Delta_o\) , determines the color of the coordination complex. According to the spectrochemical series , the high spin ligands are considered "weak field," and absorb longer wavelengths of light (weak \(\Delta_o\)) , while complexes with low spin ligands absorb light of greater frequency (high \(\Delta_o\)) . The color seen is the complementary color of the color associated with the absorbed wavelength. A sample ligand strength list is given here, but see Crystal Field Splitting for a more complete list: CN - > en > NH 3 > H 2 O > F - >SCN - > Cl - From this information, it is clear that NH 3 is a stronger ligand than Cl - , which means that the complex involving NH 3 has a greater ∆, and the complex will be low spin. Because of the larger ∆, the electrons 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, the NH 3 containing solution is predicted to be orange References Cox, P. A. Instant Notes Inorganic Chemistry . Second ed.