In a coordination compound, the so-called “crystalfield” gives rise to a differe
ID: 1758018 • Letter: I
Question
In a coordination compound, the so-called “crystalfield” gives rise to a difference in energy levels for someof the electrons in a transition metal ion. That is, electrons canoccupy one of two states, separated by the crystal field splittingenergy . The octahedral complex [Cr(NH3)6]3+ has acrystal field splitting of 0~ 2.16 eV, while[Co(NH3)6]3+ has 0~ 2.84 eV.What color are these compounds? Make use of thetable below.
If a compound absorbs a certain color of light, it exhibits thecolor complementary to the color of absorbed light.
Table 1: Absorbed wavelength and ObservedColor
(nm) AbsorbedColor........Observed color
400.........violet......................greenish - yellow
450.........blue........................yellow
490.........blue-green..............red
570.........yellow-green...........violet
580.........yellow.....................dark blue
600.........orange.....................blue
650.........red..........................green
Explanation / Answer
For these two particular compounds the electrons that occupy themetal atom's d orbtials require an energy equal to the crystalfield splitting, so that they can be excited. The source of thisenergy is from photons. [Cr(NH3)6]3+ has a crystal field splitting of0~ 2.16 eV, E = h*c/wavelength wavelength = h*c/E = (6.626e-34 Js * 3e8 m/s)/(2.16 eV * 1.60e-19J/1eV) = 5.75 e-7 m = 575 nm So it absorbs yellow light and has a blue color color while [Co(NH3)6]3+ has 0~ 2.84 eV. wavelength = h*c/E = (6.626e-34 Js * 3e8 m/s)/(2.84 eV * 1.60e-19J/1eV) = 4.37 e-7 m = 437 nm so it absorbs blue light but has a yellow color