Part 3: Crystal Field Theory: Electronic structure of d-metal complexes

Question 1

crystal field theory

Answer 1

• CFT
• model that attempts to explain electronic structure of transition metal ions in complexes and ionic solids
• not a bonding theory
• several assumptions:
  1) a ligand lone pair, neutral and charged ligand are modelled as a point negative charge
  2) the ligands are held around the central TM by purely electrostatic forces
  3) energy of d-orbitals in the complex is higher than the energy of the d-orbitals of the free M+ cation
  4) only d-electrons in the valence shell of the TM

Question 2

splitting of d-orbitals in octahedral field

Answer 2

• d-orbitals are split and classified in 2 sets: axial d-orbtials (eg; lobes are oriented along cartesian axis) and inter-axial d-orbtials (eg; lobes are oriented in between cartesian axis)
• if the electrostatic field created by the point charge ligands is octahedral, the energy of the electrons in the 3d orbitals that point directly at the ligands are raised in energy with respect to the spherical field, while the energy of the electrons in the 3d orbitals that point between the ligands are lowered in energy with respect to the spherical ligand
• eg orbitals go up in energy with respect to average because they have stronger repulsion with negative charged ligands
• eg = (3/5)∆; t2g = (2/5)∆: the amount in which the t2g orbitals go down in energy must be compensated by the amount of energy eg goes up in

Question 3

spectrochemical series

Answer 3

• these series sort ligands by splitting of d-orbitals. they are sorted in order of increased crystal-field splitting parameter
• strong-field ligands cause large splitting, while weak-field ligands cause small-splitting. these splittings result in the specific colours we see (i.e. large splitting causes us to see complimentary colours associated with higher energies, and vice versa)

Question 4

how does ∆ depend on metals?

Answer 4

larger ∆ for metals in higher oxidation states, ∆ increases down the group

Question 5

crystal field stabilization energy

Answer 5

change energy of the d-electrons with respect to the spherical distribution of negative charge

Question 6

pairing energy (P)

Answer 6

energy required when placing two electrons into one orbital

Question 7

why are two possible configurations possible for certain metals?

Answer 7

• this is due to pairing energy effect
• low-spin effect vs. high-spin effect
• if P < ∆, low-spin configuration is stable
• if P > ∆, high-spin configuration is stable
• if a ligand is a weak-field ligand, a high-spin state is expected
• if a ligand is a stong-field ligand, a low-spin state is expected
• 4d and 5d complexes have large ∆ and are always low-spin