Lecture 24: Crystal Field Theory

Read section 20.3 from your textbook.

crystal field model

  1. All interactions are electrostatic
  2. Ligands are negative point charges
  3. Electron repulsion between ligands and electron orbitals of metals


Let's think about repulsion between electrons in Cu+ and 2 ammonia ligands:



How does this relate to the molecular orbital diagram?



Follow the same process with Cu+ and 3 phosphine ligands:



Now look at the "d" orbitals or metal based orbitals in the molecular orbital diagram.



The d orbitals in a trigonal bipyramidal structure have this ordering due to electron-electron repulsion.



It's full molecular orbital diagram looks like this:



octahedral field

Let's move on to one of the most important geometries for transition metal complexes: octahedral. We say that a metal ion surrounded by 6 ligands is in an octahedral field. Imagine that the metal cation is a point charge and the ligands are anionic point charges.



The difference in energy of the d orbitals depends on the ligand. Strong field ligands cause a greater energy difference than weak field ligands.

I- < Br- < S-2 < SCN- < Cl- < N3-, F- < urea, OH- < ox, O-2 < H2O < NCS- < py, NH3 < en < bpy, phen < NO2- < CH3-, C6H5- < CN- < CO

The splitting of d orbitals is also a function of the metal. It is less in complexes of first row elements than in complexes of second and third row transition metals.

tetrahedral field

The splitting of d orbitals in a tetrahedral field is less than (4/9) the splitting with the same ligands in an octahedral field.