13.1.5 Colour in Transition Metals

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

Crystal Field Theory (CFT)

• The crystal field theory is a model based on electrostatic point charges and is used to explain colour in transition metal compounds
• In a transition metal atom, the five orbitals that make up the d-subshell all have the same energy. The term for this is degenerate
• However, when ligands are attached to a transition metal ion, the electric field formed by the lone pairs of electrons on the ligands repel the electrons in the d-subshell causing the d-orbitals to split in energy
• The dative bonding from the ligands causes the five d orbitals to split into two sets
• These two sets are not equal in energy and are described as being non-degenerate orbitals

Upon bonding to ligands, the d orbitals of the transition element ion split into two non-degenerate sets of orbitals

Splitting in octahedral complexes

• In octahedral complexes, there are six ligands arranged around the central metal ion
• The lone pairs of the ligands repel the electrons in the x²-y² and z² orbitals of the metal ion more than they repel the electrons in the 3d_yz, 3d_xz, and 3d_xy orbitals
• This is because the 3d_x2-y2 and 3d_z2 orbitals line up with the dative bonds in the complex’s octahedral shape
• The incoming ligands are attached to or approaching the central metal ion along the x, y and z axes, and the 3d_x2-y2 and 3d_z2 orbitals have lobes along these axes
• The electrons in these two orbitals are closer to the bonding electrons, so there is more repulsion
• This means that when the d orbitals split, the 3d_x2-y2 and 3d_z2 orbitals are at a slightly higher energy level than the other three
• The difference in energy between the non-degenerate d orbitals is labelled as ΔE

Diagram showing the shapes and orientation of the five d-orbitals

Absorption of light

• When white light passes through a solution of aqueous nickel(II) sulfate an electron in the lower energy d-orbitals is excited and jumps up into the higher energy d-orbitals
•  A photon of red light is absorbed and light of the complementary colour (green) is transmitted
• This is why nickel(II) sulfate solution appears green
• The energy of the separation is ΔE corresponding to a wavelength of about 630-700 nm

Electron promotion in a Ni(II) complex when light shines on the solution