Coloured ions

• Most transition metal compounds appear coloured because they absorb energy corresponding to certain parts of visible electromagnetic spectrum.

  • The colour seen is part of the visible spectrum that aren't absorbed e.g. a red compound will absorb all frequencies of the spectrum apart from red light
• Light can be either be transmitted or reflected through or off an object
• Since the d sub level is only partially filled. This means that electrons can move between orbitals
• In transition metal atoms all of the d sub level orbitals sit at the same energy level but when an ion or compound is formed, the energy of the orbitals differ
• The difference in energy between the d sub levels orbitals can be called ∆E
• The frequency of light which provide the ∆E can be found using the following equation ∆E = hv

  • h is plank constant
  • v is the frequency of light
• To work out frequency of light you can use the following equation v = c ÷ λ

  • v is the frequency of light
  • c is the speed of light
  • λ is the wavelength
• Factors affecting colours of transition metals:

  • Size of ligands
  • Type of ligands
  • Coordination number
  • Oxidation number
  • The transition metal ion
• Ultraviolet-visible spectroscopy (UV - Vis) can be used to determine the concentration of a transition metal complex solution.

  • It passes light through a filter and then through a sample. Some of the light is absorbed while the rest pass through
  • Then a colorimeter is used to measure the absorbance of the sample. The amount of light absorbed is proportional to the concentration of the absorbing species
• Method to determine unknown concentration of transition metals

  1. First, add an alternative ligand to intensify the colour e.g. thiocyanate (SCN-) or bipyridyl
  2. Then, make up solutions of known concentrations
  3. Measure the absorption of unknown concentration
  4. Plot calibration curve
  5. Test unknown and use calibration curve to find the concentration
• Red = Low energy light

  Orange |

  Yellow |

  Green |

  Blue |

  Indigo ↓ 

  Violet = High energy light
• If a transition metal complex has a large ∆E between d orbitals, it will absorb light from the high energy level so it appears red, orange or yellow as light from low energy level will be transmitted / reflected
• If a transition metal complex has a small ∆E between d orbitals, it will absorb light from the low energy level so it appears blue, indigo or violet as light from high energy level will be transmitted / reflected