Colour in Aqueous ions
Perception of colour
- Most transition metal compounds appear coloured. This is because they absorb energy corresponding to certain parts of the visible electromagnetic spectrum
- The colour that is seen is made up of the parts of the visible spectrum that aren’t absorbed
- For example, a green compound will absorb all frequencies of the spectrum apart from green light, which is transmitted
- The colours absorbed are complementary to the colour observed
The colour wheel showing complementary colours in the visible light region of the electromagnetic spectrum
- Complementary colours are any two colours which are directly opposite each other in the colour wheel
- For example, the complementary colour of red is green and the complementary colours of red-violet are yellow-green
Splitting of 3d energy levels
- In a transition metal atom, the five orbitals that make up the d-subshell all have the same energy.
- Ions that have completely filled 3d energy levels (such as Zn2+) and ions that have no electrons in their 3d subshells (such as Sc3+) are not coloured
- Transition metals have a partially filled 3d energy level
- When ligands attach to the central metal ion the energy level splits into two levels with slightly different energies
- If one of the electrons in the lower energy level absorbs energy from the visible spectrum it can move to the higher energy level
- This process is known as promotion / excitation
- The amount of energy absorbed depends on the difference between the energy levels
- A larger energy difference means the electron absorbs more energy
- The amount of energy gained by the electron is directly proportional to the frequency of the absorbed light and inversely proportional to the wavelength
Upon bonding to ligands, the d orbitals of the transition element ion split into sets of orbitals with different energies