Lecture 6

Question 1

What is the Jahn- Teller effect?

Answer 1

Structural Distortions of Octahedral Complexes
- The Jahn-Teller Theorem:
if electrons are unequally distributed among
orbitals of the same energy, the complex
distorts so that the orbitals no longer have the same energy.

Question 2

What are the predictions made by Jahn-teller theorme?

Answer 2

Low spin complexes;
d7= LARGE distortion predicted e.g. Co2+

High spin complexes;
d4= LARGE distortion predicted e.g. Cr2+ and Mn3+
d9= LARGE distortion predicted e.g. Cu2+

Question 3

What impact does distortion have on octahedral complexes?

Answer 3

• Tetragonal compression (rare)
• Tetragonal elongation- less repulsion between apical ligands and occupied orbitals in z direction
• the effect of this ‘tetragonal’ distortion (elongation) on the energies of the d-orbitals is; dz2 orbital- energy drops
  dxz and dyz orbitals- also stabilised
  the distorted complex is more stable

Question 4

Square Planar

Answer 4

• imagine a complete removal of the 2 ligands in z direction- common with d8 metal ions, such as Ni2+
• the energy of the dz2 orbital; drops further since theres no more repulsion
• the other orbitals; with a component in z direction are also stabilised (dxz and dyz orbitals)
• no distortion predicted by Jahn-Teller
• small tetragonal distortion: no gain in CFSE
• when the energy difference Ѕ exceeds the pairing energy P, it becomes energetically favourable to form a square planar complex

Question 5

Whats the interaction between d-orbitals and the ligands in a tetrahedral complex?

Answer 5

• these 2 orbitals are fairly away from the ligands- dz2, dx2-y2
• these three orbitals are closer to the ligands- dyz, dzx, dxy
• However, none of the orbitals points directly at the ligands

Question 6

What effect do the ligands have on the d-orbital energy in tetrahedral?

Answer 6

• Fewer ligands, therefore ∆T is less than ∆o
  ∆T= 4/9∆o
• consequently; P is almost always larger than ∆T
• Tetrahedral complexes are essentially all high spin