Suggested reading: Chapter 10
Crystal field theory
Transition metal molecules have characteristic colors - there must be some low-energy electronic transitions accessible
Since transition metals differ from the other elements by the partially-filled d subshell, examine the effects of ligands on the idealised d-oribtals on the metal
Ligands have an occupied lone pair coordinating to the metal - the effect of all the ligands of the molecule is the crystal field
d orbitals on the transition metal are "split" by the effect of this field: crystal field splitting
A d orbital which is "avoiding" the lone pairs is stabilized (lower in energy), a d orbital which is "in the way of" the lone pairs is higher in energy
| reference (no crystal field) |
tetrahedal crystal field |
octahedral crystal field |
square planar crystal field |
| _ _ _ | _ _ | _ | |
| _ _ _ _ _ | _ | ||
| _ _ | |||
| _ _ _ | _ _ _ |
The energy between the upper and lower levels is the crystal field splitting energy (D) and can be measured by the energy of light emitted
The effect of ligands on D
In general, very ionic ligands lead to a small value for D, and neutral, covalent type ligands lead to a larger value for D
The higher the charge on the metal, the larger the value for D
Spectrochemical series: I- < Br- < Cl- < F- < OH- < H2O < :NCS- < NH3 < en < CN- < CO
The spectrochemical series has an effect on the way we assign electrons in the crystal field d orbital diagram
Small crystal field split: electrons are dominated by mutual repulsion - high spin
Large crystal field split: electrons are dominated by lower energy of orbitals - low spin
Most tetrahedral coordination molecules are high spin
Jahn-Teller theory
Degenerate orbitals cannot be unevenly occupied
We implicitly use Jahn-Teller theory in our description of atomic orbitals
Other example: C4H4 as a triplet and singlet (aromatic and antiaromatic rules are all a consequence of Jahn-Teller theory)
When the crystal field picture leads to a prediction that would break the Jahn-Teller rule, it is observed that the molecule distorts in order to remove the degeneracy - Jahn-Teller distortion.