13.2 Coloured complexes

• In isolated atom, d orbital has 2 energy levels. In complex ion they split into two sublevels. Electronic transitions between these sublevels leads to absorption and emission of photons of visible light.

Crystal Field Theory (CFT)

• D sub-level consists of five d orbitals d_xy, d_yz, d_xz, d_x2-y2, d_z2
• In free metal ion, Mⁿ⁺, with ligands infinite distance away, these five orbitals are degenerate (same level)
• If electrostatic field created by the ligand point charge is spherically symmetrical, energies of d-orbital will remain degenerate but increase in energy
• If electrostatic field created by ligand is octahedral, d orbitals will split into two sets of degenerate energy, the t_2g set and the e_g T_2g set will decrease in energy (stabilize). E_g will increase in energy (destabilized)
• t_2g includes d_xy, d_yz, d_xz,
• e_g includes d_x2-y2, d_z2 which point at axis ligands are on
• Crystal Field, field caused by negatively charged ligands, which cause loss of degeneracy
• After 3 spots are filled in t_2g, it can choose to go to higher energy level e_g, where destabilized. Or take more energy to fill half full t_2g This energy is called P, pairing energy.

• Factors affecting crystal field splitting energy
  1. Identity of metal ion
     1. Δ_o increases, when descending a group with metal in same oxidation state
  2. Oxidation state of metal ion
     1. Δ_o increases as oxidation state increases
  3. Nature of ligands
     1. Ammonia has greater charge density than water
  4. Geometry of complex ion
     1. Δ_t for tetrahedral complex is about 4/9 Δ_o

Explanation of colour transition metal complexes

• Colour comes from partially filled d orbital
• Transition metal complexes absorb complementary colours to what they transmit
• When electron jumps from lower to higher energy, requires photon of energy and thus emits light