Edexcel A Level Chemistry:复习笔记6.2.3 Coloured Ions

Coloured 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

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

orbital-splitting

Upon bonding to ligands, the d orbitals of the transition element ion split into sets of orbitals with different energies

Changes in Colour

The size of the splitting energy ΔE in the d-orbitals is influenced by the following four factors:

  • The size and type of ligands
  • The nuclear charge and identity of the metal ion
  • The oxidation state of the metal
  • The shape of the complex

6.2-Chemistry-of-Transition-Elements-Coloured-Transition-Metal-Complexes

The large variety of coloured compounds is a defining characteristic of transition metals

Size and type of ligand

  • The nature of the ligand influences the strength of the interaction between ligand and central metal ion
  • Ligands vary in their charge density
  • The greater the charge density; the more strongly the ligand interacts with the metal ion causing greater splitting of the d-orbitals
  • The further it is then shifted towards the region of the spectrum where it absorbs higher energy
  • As a result, a different colour of light is absorbed by the complex solution and a different complementary colour is observed
  • This means that complexes with the same transition elements ions, but different ligands, can have different colours
    • For example, the [Cu(H2O)6]2+ complex has a light blue colour
    • Whereas the [Cu(NH3)4(H2O)2]2+ has a dark blue colour despite the copper(II) ion having an oxidation state of +2 in both complexes

6.2-Chemistry-of-Transition-Elements-CopperII-Change-in-Colour

Ligand exchange of the water ligands by ammonia ligands causes a change in colour of the copper(II) complex solution

Oxidation number

  • When the same metal has a higher oxidation number that will also create a stronger interaction with the ligands
  • If you compare iron(II) and iron (III):
    • [Fe(H2O)6]2+ absorbs in the red region and appears green
    • But, [Fe(H2O)6]3+ absorbs in blue region and appears orange

Coordination number

  • The change of colour in a complex is also partly due to the change in coordination number and geometry of the complex ion
  • The splitting energy, ΔE, of the d-orbitals is affected by the relative orientation of the ligand as well as the d-orbitals
  • Changing the coordination number generally involves changing the ligand as well, so it is a combination of these factors that alters the strength of the interactions

 

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